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Towards a coherent article about functions (mathematics)

Rationale The following outline is a suggestion for prospective editors. For various reasons (stability of edits and links to related articles) I prefer not doing any editing myself, but present these notes as a resource for others. The basic rationale is that the article should center on helping the readers rather than on the personal preferences of the writers/editors. Therefore I will not start with my own preferred definition but with a reader-oriented one with the following characteristics: (a) utmost simplicity; (b) enhancing clarity by adhering to the principle of separation of concerns, in this case separating the concept of function pure and simple from characterizing a function as being from $X$ to $Y$; (c) the most general one in view of its algebraic properties, especially around composition; (d) prevalent in basic university/college textbooks in mathematics; (e) a convenient logical basis for explaining/understanding/comparing other variants. It is fortunate that all these properties happen to coincide. Also fortunate is that in the current literature there are essentially only two variants, simply distinguished by whether or not the notion of a codomain plays any role, so covering both remains very manageable. Also clarifying for the readers are brief justifications of the design decisions behind the definitions, without turning the article into a fully-fledged tutorial that is too long for Wikipedia. In view of the many misconceptions observed in the printed literature and on the web (including Wikipedia), a substantial package of references is indispensable. The text follows next. Boute (talk) 13:18, 15 February 2022 (UTC)

Outline for a new version of the article

Outline for the article (text starts here) The concept of a function or a mapping has been described (Herstein, page 9) as "probably the single most important and universal notion that runs through all of mathematics". Evidently this also pertains to all other branches of science (physics, engineering etc.) where mathematics is used. In present-day mathematics, there are essentially two major variants of the function concept, and in a balanced account both must be addressed. For this purpose, we designate them as (A) the plain variant and (B) the labeled variant, which has a codomain. The subject matter also requires ample references, also because different formulations often define the same variant, thereby clarifying each other. About a dozen paragraphs suffice for giving the reader a structured guide through the rather varied literature. A. Functions: the plain variant This variant is the simplest and also the most widespread throughout the sciences, including (but not limited to) calculus/analysis, set theory, logic, algebra, discrete mathematics, computer science, and mathematical physics. Authors and specific page numbers will be mentioned later. A.1 Basic definition One of the simplest formulations is provided by Apostol (p.53): "A function $f$ is a set of ordered pairs $(x,y)$ no two of which have the same first member." In general, a collection of ordered pairs is called a graph or a relation and is called functional or determinate if no two pairs have the same first member (or component). Thus the preceding definition can be rephrased by saying that a function is a functional graph ((Bourbaki p. 77). Formulations that are equivalent in content and style appear in calculus/analysis (Apostol p. 53, Flett, p. 4), set theory (Bourbaki p. 77, Dasgupta p. 8, Quine p. 21, Suppes p. 57, Tarski & Givant p. 3), logic (Mendelson p. 6, Tarski p. 98), discrete mathematics (Scheinerman p. 73), computer science ((Meyer p. 25, Reynolds p. 452). The wordings differ but all define the same concept, apart from the fact that some authors apply them to classes instead of mere sets. A.2 Conventions The set of all first members of the ordered pairs in a graph (or relation) $G$ is called the domain of $G$ and is written $\mathrm {dom} \,G$ or ${\mathcal {D}}\,G$. The set of all second members is called the range of $G$ and is written $\mathrm {ran} \,G$ or ${\mathcal {R}}\,G$. Let $f$ be a function (functional graph). For each $x$ in the domain of $f$ there is exactly one $y$ such that $(x,y)\in f$. Hence $y$ is uniquely determined by $f$ and $x$. It is therefore properly called the value of $f$ at $x$ and can be unambiguously denoted by some suitable combination of $f$ and $x$, the common "default" form being $f\,x$ or $f(x)$. Other forms may be chosen as convenient by prior agreement, such as $f_{x}$ or $x^{f}$ . A common example of the latter is writing $M^{\mathsf {T}}$ for matrix transposition. A.3 The function equality theorem (Apostol p. 54) Functions $f$ and $g$ are equal ($f=g$) if and only if (a) $f$ and $g$ have the same domain and (b) $f(x)=g(x)$ for every $x$ in this domain. This theorem follows directly from set equality and holds for all formulations (preceding and following) of the definition of plain functions. It implies that a (plain) function $f$ is fully specified by its domain and the value $f(x)$ for each $x$ in that domain. An illustration follows next. A.4 Function composition This is the most important operation on functions. For any (plain) functions $f$ and $g$, the composition $g\circ f$ (also written $f\,;g$) is also a function, specified as follows: (a) the domain of $g\circ f$ is the set of all values $x$ in the domain of $f$ such that $f(x)$ is in the domain of $g$ and (b) for any such $x$, the value of $(g\circ f)(x)$ is given by $(g\circ f)(x)=g(f(x))$ or, written with less clutter, $(g\circ f)x=g(f\,x)$ (see Apostol p. 140, Flett p. 11, Suppes p. 87, Tarski & Givant p. 3, Mendelson p. 7, Meyer p. 32, Reynolds p. 450,452). Composition has the interesting property that, for all functions $f$, $g$ and $h$, we have $h\circ (g\circ f)=(h\circ g)\circ f$. This associativity allows making the parentheses optional and writing, for instance, $h\circ g\circ f=f\,;g\,;h$. A.5 Conveying domain and range information The literature presents numerous conventions for relating the domain and/or range of a (plain) function $f$ to sets $X$ and $Y$. A helpful preamble is the following legend. statement meaning $f$ is a partial function on $X$ $\mathrm {dom} \,f\subseteq X$ $f$ is a (total) function on $X$ $\mathrm {dom} \,f=X$ statement meaning $f$ is (in)to $Y$ $\mathrm {ran} \,f\subseteq Y$ $f$ is onto $Y$ $\mathrm {ran} \,f=Y$ For instance (Apostol p. 578, Flett p. 5, Dasgupta p. 10, Scheinerman p. 169, Meyer p. 26, Reynolds p. 458): A (total) function from $X$ (in)to $Y$ is a function with domain $X$ and range included in $Y$. Flett (p. 5) warns that such phrases only conveys information about the domain and the range but does not define a new kind of function. A function from $X$ to $Y$ is commonly introduced by writing $f:X\rightarrow Y$, where $X\rightarrow Y$ can be interpreted as the set of all (total) functions from $X$ (in)to $Y$ (Meyer p. 26, Reynolds p. 458), in other contexts also written $Y^{X}$. As a logical consequence, $f:X\rightarrow Y$ stipulates that (a) the domain of $f$ is $X$ and (b) the values $f(x)$ are in $Y$ and can be further specified, for instance, by a formula. This style is very convenient, as illustrated by the following function specifications ${\mathsf {sqra}}:\mathbb {R} \rightarrow \mathbb {R}$ with ${\mathsf {sqra}}\,x=x^{2}$ and ${\mathsf {sqrb}}:\mathbb {R} \rightarrow \mathbb {R} _{\geq 0}$ with ${\mathsf {sqrb}}\,x=x^{2}$. By definition, both specify the same function (${\mathsf {sqra}}={\mathsf {sqrb}}$) which is onto $\mathbb {R} _{\geq 0}$ but not onto $\mathbb {R}$. Consider also ${\mathsf {posrt}}:\mathbb {R} _{\geq 0}\rightarrow \mathbb {R} _{\geq 0}$ with $({\mathsf {posrt}}\,x)^{2}=x$ and ${\mathsf {negrt}}:\mathbb {R} _{\geq 0}\rightarrow \mathbb {R} _{\leq 0}$ with $({\mathsf {negrt}}\,x)^{2}=x$. Here ${\mathsf {posrt}}$ and ${\mathsf {negrt}}$ are respectively the positive and negative square root function. Both are functions from $\mathbb {R} _{\geq 0}$ to $\mathbb {R}$ but ${\mathsf {posrt}}$ is onto $\mathbb {R} _{\geq 0}$ whereas ${\mathsf {negrt}}$ is onto $\mathbb {R} _{\leq 0}$. Similarly, a partial function from $X$ to $Y$ is a function with domain included in $X$ and range included in $Y$. For instance, in calculus/analysis most functions are defined on some subset (interval, region, ...) of $\mathbb {R}$, $\mathbb {R} ^{n}$, $\mathbb {C}$, $\mathbb {C} ^{n}$ and so on hence are partial on these sets. For the set of partial functions from $X$ (in)to $Y$ one finds various notations, such as $X\not \rightarrow Y$ (Meyer p. 26) and $X\;{\stackrel {\longrightarrow }{\scriptscriptstyle \mathrm {PFUN} }}\;Y$ (Reynolds p. 458). As a very interesting illustration, the reader can verify that, given $f:X\rightarrow Y$ and $g:U\rightarrow V$, the composition $g\circ f$ is a partial function from $X$ to $V$ and that $g\circ f$ is a total function from $X$ to $V$ iff $\mathrm {ran} \,f\subseteq \mathrm {dom} \,g$, which trivially holds in case $Y=U$. Important remark: as in natural language, onto is used as a preposition, mentioning $Y$ explicitly (Flett p. 5, Scheinerman p. 172; more references follow in the next paragraph). A function that is onto $Y$ is sometimes called surjective on $Y$ or a surjection on $Y$. Scheinerman (p, 172) designates omitting $Y$ as "mathspeak", but it is not harmless and may cause misunderstandings. A.6 A shortcut formulation for a function from $X$ to $Y$ Quite a few authors (Bartle & Sherbert p. 5, Royden p. 8, Halmos p. 30, Herstein p. 10, Gerstein p. 110, Gries & Schneider p. 280, Szekeres p. 10) do not start from the basic definition given earlier but directly define a function $f$ from $X$ (in)to $Y$ as a subset of $X\times Y$ such that for every $x$ in $X$ there is exactly one $y$ in $Y$ such that $(x,y)\in f$. Less often, some authors (Bartle p. 13, Gries and Schneider p. 280) use a formulation that amounts to replacing "exactly one" by "at most one", which effectively defines a partial function from $X$ to $Y$. Important remark: appearances notwithstanding, this shortcut formulation logically defines exactly the same kind of function as the basic definition with exactly the same properties and conventions. In particular:, The function equality theorem holds as stated (only mentioned explicitly by Gerstein p. 113). Composition $g\circ f$ is defined for any functions $f$, $g$, although some authors overlook this and define $g\circ f$ only for $f:X\rightarrow Y$ and $g:Y\rightarrow Z$, which reduces generality by assuming $\mathrm {ran} \,f\subseteq \mathrm {dom} \,g$. Any function $f$ is a function from its domain to any superset of its range. Hence the versatile specification style illustrated by the examples ${\mathsf {sqra}}$, ${\mathsf {sqrb}}$, ${\mathsf {posrt}}$, ${\mathsf {negrt}}$ remains applicable. As before, onto-ness is specified with respect to a set, using "onto" as a preposition (Bartle p. 13, Bartle & Sherbert p.7, Royden p. 8, Halmos p. 31, Herstein p. 12, Gerstein p. 118, Szekeres p. 11). A.7 Separating the plain function concept from its graph Whereas defining a function as a graph is very precise and rigorous, it creates some ambiguities for certain common conventions. Just two examples: (i) writing $f^{n}$ for $n$-fold function composition and $S^{n}$ for the $n$-fold Cartesian product, and (ii) defining sequences (in particular pairs) as functions on some subset of the natural numbers. Some definitions (Carlson p. 182, Kolmogorov & Fomin p. 5, Rudin p. 21) avoid this by defining a function $f$ from $X$ to $Y$ less formally as associating "in some manner" a unique value $f(x)$ in $Y$ with every value $x$ in $X$, called the domain of $f$. This can be captured as follows: A (plain) function is an entity that is fully specified by a domain, which is a collection (set or a class) of values, and by a unique value assigned to each element in this domain. As noted by Royden (footnote p. 8) this formulation can be made precise by taking the statement of the function equality theorem (A.3) as an axiom. The range of $f$ is then the set of all values $f(x)$ for $x$ in the domain of $f$. All earlier auxiliary formulations carry through literally as stated, namely, fully general composition (A.4) and conveying domain/range information (A5). The graph of $f$ is then the set of all pairs $(x,fx)$ for $x$ in the domain of $f$ and is denoted by ${\mathcal {G}}\,f$. Evidently $f=g$ if and only if ${\mathcal {G}}\,f={\mathcal {G}}\,g$. This may be useful in simplifying certain proofs and definitions (e.g., for inverses). B. Functions: the labeled variant and the notion of codomain Recall that, for plain functions, the appearance of $Y$ in $f:X\rightarrow Y$ specifies that $f(x)\in Y$, without making $Y$ an attribute of $f$ (in contrast $X$, which is specified to be the domain). How to exploit this flexibility in function specifications was demonstrated by the examples ${\mathsf {sqra}}$, ${\mathsf {sqrb}}$, ${\mathsf {posrt}}$, ${\mathsf {negrt}}$. Dasgupta (p. 10) points out that making $Y$ an attribute of $f$ in a proper fashion requires explicitly attaching $Y$ to $f$ to form a triplet $\langle f,X,Y\rangle$. Mac Lane (p. 27) calls this modification labelling. In general, A (labeled) function is a triplet $F:=\langle f,X,Y\rangle$ where $f$ is a (plain) partial function from $X$ to $Y$. The set $X$ is called the source of $F$ and $Y$ is called the target of $F$ or the codomain of $F$. The domain and the range of $F$ are those of $f$. Similar formulations, sometimes identifying domain and source, are given by Bourbaki p. 76, Adámek & al. footnote p. 14, Bird & De Moor p. 26, Pierce p. 2. Some of the major differences with the plain varianr are: Equality of labeled functions requires equality for source, domain, codomain and images. For a labeled function $F:=\langle f,X,Y\rangle$, the following terminology holds: (i) $F$ is partial means that $\mathrm {dom} \,f\subseteq X$, (ii) $F$ is total means that $\mathrm {dom} \,f=X$, and (iii) or $F$ is surjective means that $\mathrm {ran} \,f=Y$. Composition $G\circ F$ is defined only in case $\mathrm {cod} \,F=\mathrm {src} \,G$. In case $G$ is total this means that $\mathrm {cod} \,F=\mathrm {dom} \,G$, which is quite restrictive when compared to the plain variant. References Last updated: Boute (talk) 13:18, 15 February 2022 (UTC)

Any hidden (collapsed} text needs a visible summary or abstract. Summary This proposal is a resource for editors to improve the article. It starts with the simplest and most widely used definition (in essentially two formulations), which also serves as the basis for explaining more complicated ones. Function composition receives special attention because it is the most important operation on functions. Various misconceptions are resolved, especially regarding codomains. About 30 literature references support the information given. These sources are selected for reliability but, whenever possible, also for being easy to find. Boute (talk) 05:35, 17 August 2022 (UTC)

Discussion

I have collapsed the proposal for distinguishing easily this long proposal from the discussion about it.

This proposal contains interesting ideas. However, the present article result from a consensus involving many editors. It is written for beginners in mathematics, who must not be confused by technical considerations that are outside their knowledge. At a first glance this has not being considered by the author of the proposal. So, IMO, the proposal can be useful for improving some points of the article and sourcing, but not is not worth to be expanded into a new version of the article. D.Lazard (talk) 15:20, 15 February 2022 (UTC)

With very few exceptions, all material in the proposal was written to be understandable by beginners. A good indication of the level is that all material comes from properly referenced introductory textbooks. Moreover, the material is presented in such a sequence that the interested reader can stop at any time, even (in the extreme) after the first paragraph with the initial definition. I understand the complaint about length of the proposal, but in reality the proposal is quite short in comparison with the reality it must properly reflect. Don't forget that the article about functions is considered "vital". The definition in the current Wikipedia has been shown to be logically defective and definitely does not reflect the view on functions prevalent in the open literature, as demonstrated by the references. Talking about codomains before the simpler variant has been presented is about the most confusing thing one can do to beginners. Unfortunately, in half a dozen randomly selected introductory textbooks that attempt to introduce the concept of codomain, this causes a logical error (contact me by email for the references). Boute (talk) 08:30, 16 February 2022 (UTC)

I disagree definitively with your proposal:

Your "plain" definition implies that the range of a function is a set. So, it is impossible to specify a function without specifying first a set that contains the range. This is this set that is called the codomain. So, in the practice of most users, only "labeled" functions are considered. Moreover, if emphasis is put on "plain" functions, there is no more concept of a surjective function. So, emphasizing on "plain" functions, or giving the same weight to both definitions may confuse most readers.

You assert that you have found a logical error in the article (more exactly in textbooks that follow the same approach). You must be more explicit and open a discussion about this alleged error.

As far as I know, the "plain" definition is used only in mathematical logic and related area. This seems the opinion of the authors of Wikipedia article, as this definition appears only in § In the foundations of mathematics and set theory and § In computer science (with the mention of lambda calculus). Nevertheless, these mention could deserve to be expanded, maybe by adding a new section for comparing and discussing the two definitions. But, again, only one definition must appear in the main part to the article. D.Lazard (talk) 09:37, 16 February 2022 (UTC)

The "plain" definition is the one used in all 25 references cited for that variant in the proposal, covering many branches of science. Please look at them before making unfounded statements. None of these sources mention a codomain at all, since it is not needed for that variant. What you claim to be "impossible" (in a non sequitur) is in fact entirely normal. The logical errors in the current Wikipedia definition were clearly explained in my comments on these talk pages last month (see here). Not looking at the references and disregarding logical analysis is the main cause of confusion in these discussion pages. Boute (talk) 10:06, 16 February 2022 (UTC)

First of all, I want to thank Boute for the valuable material, in particular the sources, in the outline. I feel most (or even all) of them should be used (I'd move some of them to other articles, like function composition), but this will take some time, especially since it is adressed to talk page readers, not to article readers (WP:EPSTYLE).

I also agree with Boute that the definition in the article, while (I believe) trying to explain the concept of labeled function, fails to do so properly. I suggest to change at least section Function_(mathematics)#Definition to Given two sets X and Y, a function f from X to Y is an assignment of an element of Y to each element of X. X is called the domain of f and Y is called the codomain of f. This would fix Boute's above argument ("ad D"). I'd also apply the same change to the lead, but this can be discussed separately. (By the way: In the "Definitions" section, I'd like to see a formal version using ∀∃, or at least an English sentence without any ambiguity w.r.t the quantification order; cf. also User_talk:Jochen_Burghardt#Function_(mathematics),_"one"_or_"an".)

As for the discussion about "plain" and/or "labeled" version, I am, however, inclinded towards a presentation that is "abstract", i.e. representation-independent, as long as possible. The concept of a function is pretty much abstract, and can be understood, and applied, independent of its "implementation" in set theory. As an analogy: the concept or ordered pair is usually introduced by "specifying" just its properties; after that, remarks on possible implementations may, or may not, follow. (The notion of "specification" and "implementation" are borrowed from formal verification in computer science.) In category theory and in universal algebra, (labeled) functions are used, under the name "morphism" and "homomorphism", respectively, without referring to the set implementation. Also the "dynamic" aspect of a function, illustrated by File:Function_machine2.svg, fits well with an abstract description (e.g. "put some string in, get some integer out", no need to know Cartesian products), without needing an understanding of the set implementation. However, Boute's outline (necessarily) starts with implementation details almost from the beginning.

Nevertheless, both plain and labeled implementations should be presented in the article, with their interrelations and discrepancies. I'm not sure about how to achieve this. Maybe, the section Function_(mathematics)#Relational_approach could be split into "Relational approach (plain)" and "Relational approach (labeled)", and maybe even accompanied by "Algebraic approach" (mentioning homomorphisms and morphisms as primitives - an expert should comment on this)? - Jochen Burghardt (talk) 13:12, 16 February 2022 (UTC)

I agree with Jochen Burghardt that, ideally, we should define a function by its properties rather than by its representation as a (functional) set/class of pairs (FSOP). This is why item A.7 in my proposal separates the two. However, as I mentioned in the "rationale", the article should serve the interests of the readers rather than our own preferences. A central issue is the order of presentation. There are many reasons for starting with the plain variant. It is the simplest, and most represented throughout the sciences, so readers will encounter it most often. Composition is defined most generally (for any pair of functions), which explains the absence of the labeled variant in calculus texts. It supports the easiest way to describe the labeled variant (as a triplet with a plain function as one of its components). I have not yet heard any arguments for starting with the labeled variant, which seems backwards. In any case, the definition should be properly referenced. The current Wikipedia formulation is incorrectly attributed to Halmos, who really says on page 30

"If $X$ and $Y$ are sets, then a function from (or on) $X$ to (or into) $Y$ is a relation $f$ such that $\mathrm {dom} \,f=X$ and such that, for each $x$ in $X$ there is a unique element $y$ in $Y$ with $(x,y)\in f$."

Clearly the plain variant. For defining the labeled variant it does not suffice adding something like "Given sets $X$ and $Y$". These sets must be attached explicitly to the function in the definition, as in a 1975 paper by Hilary B Shuard : " A function $f:A\rightarrow B$ consists of two sets $A$ and $B$ and a rule which attaches to each $x\in A$ exactly one $f(x)\in B$.". This is clearly the labeled variant. With any formulation for this variant, the extra complication of attaching the set $B$ to the function must be justified to the satisfaction of the discerning reader. No such justification can be found in the literature, apart from the circular "to satisfy a certain axiom", which shifts the burden of justification to the axiom. Aside: even category can be generalized by omitting the disjointness axiom for hom-sets (Mac Lane's book, last sentence page 27), which eliminates the need for codomains and frees composition from unnecessary restrictions (this evidently works for the axiomatic formulation of category theory; defining a category as a graph is more rigid since it requires unique endpoints for each arrow). Even so, the article we are discussing is about functions, not category theory, so we should not add exta sources of confusion. Boute (talk) 09:01, 17 February 2022 (UTC)

Your opinions as "complication", or "limiting" below, or "burden", are just your opinion and not objective qualities of the two definitions. The choice of going for the set theoretical definition (what you call plain variant) or for the morphism (labeled) has advantages or disadvantages depending on what you want to do with the concept of function. Lets also note that a large volume of literature defines function and doesn't really pay attention to the distinction. Even in your quote from Halmos, which you consider "clearly the plain variant", you see him say "or into Y". That doesn't make sense. A function defined as a relation looses the "into Y information". With the definition of function as a relation, there is no notion of surjectivity and codomain. Thatwhichislearnt (talk) 15:54, 1 February 2024 (UTC)

[As a matter of policy, I stop posting anything further here (lost effort), but will gladly reply to personal emails.]

If you read my earlier posts (open the green box in this Talk section), you will see plenty of arguments, not just an "opinion". Better still, send me an email and you will receive an itemized list justifying the term "burden". Boute (talk) 09:18, 14 February 2024 (UTC)

No thanks. Opinions about definitions are only philosophical, not math. It is not my interest. I also have not doubt that every argumentation about the supposed "burden" is only a lack of understanding of how viewing a function as a morphism, instead of as a relation, can help express information (the most immediate one being the codomain). Again, choices of definition depend on context. I suppose when talking to students in an introduction to calculus you would like to talk about "the sine function" and not pay attention to the codomain. They already have many other new theoretical detail to pay attention to. However, when doing algebraic topology you do want to pay attention to whether you are lifting a map across an inclusion function, for example. Thatwhichislearnt (talk) 15:03, 14 February 2024 (UTC)

Alas, (today) the definition stated in the article is still self-contradicting. Moreover, the citations do not match the statement. Halmos says something quite different (not bogged down by codomains). The cited reference in the encyclopedia also says something quite different (introduces codomains without contradiction) Boute (talk) 03:47, 16 July 2022 (UTC)

I noticed that Halmos's definition is different from the article. Why not go with his definition (copied below)?

If X and Y are sets, a function from (or on) X to (or into) Y is a relation f such that dom f = X and such that for each x in X there is a unique element y in Y with (x, y) $\in$ f

174.112.98.128 (talk) 17:42, 10 May 2023 (UTC)

As you suggest, Halmos's definition would indeed be a good start. Apostol uses even fewer words: "a function is a set of ordered pairs no two of which have the same first component". Sets might be generalized to classes here. To answer your question: many wikipedia editors are rather adamant to include the notion "codomain" right from the start, apparently just because the word exists. Technically the codomain notion is very limiting in many ways (starting with function composition) and in textbook definitions it is a source of logical contradictions. Boute (talk) 07:36, 5 June 2023 (UTC)

It is actually the other way around. It is very common for books to start with the set theoretical definition of function, just by imitation, to then mindlessly consider the notions of surjectivity and codomain. What leads to contradictions is never the choice of definition, but not paying attention to the choice that was made. Also "function composition", the is no problem with either definition? If anything making the co-domain part of the function makes one conscious of inclusion mappings, instead of tacitly ignoring them. Thatwhichislearnt (talk) 16:06, 2 February 2024 (UTC)

Addendum Taking a brief look at how this article fared in the past two years, I still see the same defects. In particular, let f be a function from X to Y according to the current Wikipedia definition. Now let U be any subset of X and V be any superset of Y. Then f maps every element of U to an element of V, so its domain is U and its codomain is V with the current Wikipedia definition. So both domain and codomain are in fact ill-defined. Boute (talk) 11:49, 12 February 2024 (UTC)

From what I can tell, the current definition has the domain and codomain as part of the specification of a function. If you change the domain and codomain, you get a different function. –jacobolus (t) 13:16, 12 February 2024 (UTC)

Look at the predicate after the verb "is" in "is an assignment of an element of Y to each element of X". If you change to $U\subset X$ and $V\supset Y$, it is no longer "to each element of X". So, the domain is uniquely determined by what the the function is. The codomain is the one that can be argued is not determined by what the function is in that definition, since keeping X and changing to $V\supset Y$ the function still is what it was before. I think the statement was kept somewhat informal, such that one can interpret either way regarding whether the codomain is or not part of the function. Thatwhichislearnt (talk) 14:28, 12 February 2024 (UTC)

The current (Wikipedia) definition does not say "is an assignment" but "assigns", which muddles things. But let's take your wording, because it clearly separates the definiendum ("A function from X to Y") from the definiens ("is an assignment etc."). The definiendum cannot make X and Y part of the function, even if you start with "Let X and Y be sets. A function from X to Y is an assignment etc.", Anyway, what I wanted to express with my brief addendum is my low expectation that a simple and logically tight definition will ever emerge in this article, unless someone honestly takes into account the actual literature, starting with the many references I provided 2 years ago. In the current article, even the reference to Halmos is mistaken. I'll sign off now, and am curious about whether any progress will be visible in 2026. Boute (talk) 01:11, 14 February 2024 (UTC)

That's not muddling, and it's not a definition. It's saying what a function does. Saying what a function is requires more assumptions, and the lead of an article like this is not the place for precise rigorous definitions. —David Eppstein (talk) 02:26, 14 February 2024 (UTC)

[As a matter of policy, I will not post anything further here (a waste of time), but will gladly reply to personal emails.]

As observed by David, the current text is indeed not a definition, but this is precisely the reason why it fully deserves being called "muddling". Even if one is against rigor in the lead of an article, one should not accept a situation that causes self-contradictions or ill-defined concepts that require extensive provisos and repair work afterwards. The choice is this: do we serve the readers, or some hidden agenda (self-interest, intellectual investment in one's own research or published books etc.)? Googling "criminal algebraists-axiomatisators") reveals Arnold's healthy warning against the latter attitude. Serving the readers is best achieved by starting with simplicity without sacrificing precision. An excellent principle is separation of concerns (Dijkstra), in this case: defining function without starting with "from X to Y; such qualifications can be introduced afterwards. For instance, although it is not my preferred definition (it is too concrete), I quote from Apostol, Flett and various others: "A function is a set of ordered pairs no two of which have the same first component". Boute (talk) 09:08, 14 February 2024 (UTC)

The set theory definition of function is not the only definition of function. It is just a fact of life that at this current moment of time, there are two inequivalent definitions of function. One that determines the codomain, one that doesn't. Both determine the domain. An encyclopedia should inform on that. None of those definitions is more adequate than the other. It is just a choice depending on needs, the context, or preference. The set theory definition is older, the definitions that make the codomain part of the function fit better with the more recent push to present mathematics from a category theory point of view. At the end of the day, it doesn't really matter as long as one is consistent. Wikipedia only needs to mention both, select one for the rest of the article, and be consistent to it. Thatwhichislearnt (talk) 14:49, 14 February 2024 (UTC)

set theory definition is older – the set theory definition dates more or less from the late 19th century, but didn't become widely adopted until the 20th century sometime. Our page History of the function concept does a decent job of covering this the history through the mid 20th century (the parts about recent history could use further elaboration). –jacobolus (t) 15:57, 14 February 2024 (UTC)

An category theory from the mid to second half of the 20th. What does your post imply? I don't understand. I myself cannot improve the History section, if it needs to be improved. I don't have relevant literature. All I meant to say is that while some have personal affection for the set theory definition, it also has inadequacies when you move it to more modern contexts. Who knows if in 200 years everyone will be including the codomain or not as part of the function. There is little point in fighting about definitions. Thatwhichislearnt (talk) 16:16, 14 February 2024 (UTC)

@Thatwhichislearnt In reality, both definitions, i.e., the Bourbaki function (a triple: f=(X,Y,G)) and the function as a set of pairs (binary relation), are related in such a way that the latter definition is a restriction of the former to only the graph.

I just submitted this in my last edit HERE (discussion - here) - which was reverted by a group of people who refuse to fix the article, stubbornly clinging to its old version, their own view that a function is just a binary relation. Kamil Kielczewski (talk) 14:24, 12 March 2024 (UTC)

Note that both definitions are in use, and it is not that one definition is right and the other is wrong. One can work with either one just fine, as long as what one says about functions later is adjusted accordingly. Wikipedia is (or should be) a reflection of current primary sources. Because currently there are sources for both, probably Wikipedia should mention both. One thing that is definitely wrong in the article is the definition in the section Formal Definition. It is malformed. It is not a definition. It has the same structure as "A prime number with hophops, is a natural number that is greater than 1 and not a product of smaller natural numbers", where "with hophops" plays the role of the undefined "with domain X and codomain Y". It is a predicate that the purported definition leaves undefined. In that sense, I do agree that your edit was a better option. Thatwhichislearnt (talk) 15:14, 12 March 2024 (UTC)

@Thatwhichislearnt Note that both definitions are in use, and it is not that one definition is right and the other is wrong I agree with you (although I used to think differently) - and this is reflected in the change I made - as I added a section "Reduced formal definition" in which there is an introduction where I wrote that both definitions are commonly used. Kamil Kielczewski (talk) 15:25, 12 March 2024 (UTC)

@Thatwhichislearnt one more thing - as a justification for reverting (10:00, 11 March 2024) my edit, D.Lazard wrote

"Here is a clear consensus on the talk page against this change"

which seems not to be entirely consistent with the truth, as confirmed by your opinion that this edit is an improvement to the article. Kamil Kielczewski (talk) 15:43, 12 March 2024 (UTC)

I don't know when the revert happened in relation to other discussions in the talk page. I did see that some of the objections that were of mathematical nature were unfounded. There were objections on complexity that I could get behind. In my opinion, for the section Formal Definition, I would take a bit of complexity over what is written right now that is not a definition. Thatwhichislearnt (talk) 16:59, 12 March 2024 (UTC)

@Thatwhichislearnt I will also ask other participants what they think about my reverted change in the article (related to the formal definition). If it were possible to reintroduce it, after consulting with other authors, it would be a step in the right direction and a starting point for further improvements ( that you mentioned). Do you think that would be ok? Kamil Kielczewski (talk) 20:37, 12 March 2024 (UTC)

You are reading the first paragraph, which doesn't necessarily need to be precise. I quoted from the section called Definition, where one would expect to have more details. In this article, there is one further step going to the section called Formal definition. In both of those the definition of choice does determine the domain, and doesn't determine the codomain. That's all. Thatwhichislearnt (talk) 14:39, 14 February 2024 (UTC)

Formal definition - replacement

The current definition is overcomplicated, and at the same time jagged and incomplete, and seems not to meet the requirements of modern mathematics.

This is because

• the concept of relationship was introduced unnecessarily (it has its uses, but here it is off topic)
• a function seems to be a fragmented entity (we actually don't know what it is) that has some realtion and may be domain and codomain
• in old definition is written: "function is ... a relation" - if so we will not be able to say whether the function is surjective or not - because we must have information about the codomain (relation not contains such information) (actually, based on the old definition, it is not clear whether a function is just a relation or maybe "something" else that also includes a domain and codomain)

Let us introduce the following, more direct, simple, complet and uniform "new definition" (it is actually not new concept):

Solution - new formal definition

A function $f=\{X,Y,W\}$ is a Set consisting of following elements:

• domain $X$ that is any set
• codomain $Y$ that is also any set
• graph $W\subseteq X\times Y$ being a set of pairs such that $\forall x\in X,\exists !y\in Y:(x,y)\in W$

where $\exists !$ means "there is exactly one"

Notation

In traditional notation we usually separate domain and codomain definitions from graph e.g.: "The function $f\colon X\to Y$ is given by the formula $f(x)=\ ...$" (formula represents graph W). By new definition this notation just describes following set: $f=\{X,Y,\{(x,f(x)):x\in X\}\}$. Of course, using new definition we not need to change traditional notation.

If you omit to provide the domain and codomain for a given function in traditional notation, it means that this information must be derived from the graph or context - which is often the case (and justifies separating the definition of the graph in the notation).

Examples

Lets look on following four examples with similar graphs:

• $f\colon \mathbb {R} \to \mathbb {R} ^{+},f(x)=x^{2}$ , by definition it is: $f=\{\mathbb {R} ,\mathbb {R} ^{+},\{(x,x^{2}):x\in \mathbb {R} \}\}$
• $g\colon \mathbb {R} \to \mathbb {R} ,g(x)=x^{2}$ , by definition it is $g=\{\mathbb {R} ,\{(x,x^{2}):x\in \mathbb {R} \}\}$ )
• $h\colon \mathbb {R} ^{+}\to \mathbb {R} ^{+},h(x)=x^{2}$ , by definition it is: $h=\{\mathbb {R} ^{+},\{(x,x^{2}):x\in \mathbb {R} ^{+}\}\}$
• $k\colon \mathbb {R} ^{+}\to \mathbb {R} ,k(x)=x^{2}$ , by definition it is: $k=\{\mathbb {R} ^{+},\mathbb {R} ,\{(x,x^{2}):x\in \mathbb {R} ^{+}\}\}$

distinction:

• f is not g because g has 2 elements (domain = codomain) and f has 3 elements (so f and g must be different sets)
• f is not h because h has 2 elements
• f is not k because f contains element that contains pair (0,0) - the k not contains element with such pair
• g is not h because it not contains $\mathbb {R} ^{+}$ element .
• g is not k because k has 3 elements
• h is not k because k has 3 elements

properties that can be easily derived from the new definition:

• f is surjective (note that if you consider only graph (relation) then you can't check this for f and g)
• h is a bijection
• k is injective

Conclusion

New definition:

• defines functions as a set - a basic mathematical concept - and not as a fragmented "something"
• explicitly defines a codomain which allows us to determine whether the function is surjective
• does not introduce redundant concepts (relation - Occam's razor)
• is clean, intuitive and simple

— Preceding unsigned comment added by Kamil Kielczewski (talk • contribs) 13:14, 23 February 2024 (UTC)

Please sign your posts on the talk page with four tildes (~~~~). Also, since your post is answered, you must not modify it; instead, you should open a new post after the answer.

The new formal definition you suggest is incorrect, since in a set, the elements are not ordered. So, if $\{X,Y,W\}$ would be a set, one could not distinguish between the domain and the codomain. Moreover, the phrasing "a function $f=\{X,Y,W\}$ is ..." is incorrect, as nobody writes a function this way.

Even if it would be corrected, your definition would consist essentially in replacing the definition "A binary relation between two sets X and Y is a subset W of the set of all ordered pairs (x, y) such that $x\in X$ and $y\in Y$" by the formula $(X,Y,W).$ This may be shorter to read, but makes certainly reading more difficult. Note also that in all formulations, the domain and the codomain are parts of the definition of a function; so, your main concern is wrong, that the given definition would not allow defining a surjection. D.Lazard (talk) 14:53, 23 February 2024 (UTC)

1. You write " if {X,Y,Z} would be a set, one could not distinguish between the domain and the codomai" - this is false statement. Set {X, Y, W} (which is simpler structure than (X,Y,W)) where W is subset of $X\times Y$, contains enough information to deduce what element is graph, what is domain and what is codomain (I even give some "edge" examples which shows that). This is NOT NOTATION. This is DEFINITION - precise, avoiding redundancy, using the simplest possible concepts - but still concise and short.

2. "nobody writes a function this way." - untrue. Usually everybody use traditional handy notation - but not confuse notation with definition. Counterexample: e.g. my topology professor sometimes use this definition instead traditional notation to show some sophisticated things.

3. As I wrote in the proposed change, the old definition is formulated in such a way that it is not known what a function actually is (it is fragmented "something") - which only proves that the old definition is simply weak and leads to errors (because even in the old definition itself it is written: "A function with domain X and codomain Y is a binary relation R between" (this means that function is binary relation - but binary relation not contains information about function codomain - so this is false/confusing statement - (4 examples I give also shows that binary relation ("graph") is not enough). With old definition - the reader is left with a puzzle: function is binary-relaton or not - it is somehing more?

4 You try to mix old definition witch new and write "This may be shorter to read, but makes certainly reading more difficult." - no. Just look on old definition, and look on new definition separately (dont't mix them) - the new one is simpler, more clean and more intuitive (because whe use concept of set of pairs in direct way and set simple intuitive condition to that set). My proposition is to replace old definition with new one (remove old content of "Formal definition" except first pharagraph which can be optionally moved to the end of previous section - for clarity - to not to disturb the definition section witch meta info) - so no update old definition; no mix them; Optionally below new definition we can add explanation how to read quantifiers symbols using "natural language".

5. Apart from the other arguments I presented in the proposal to which you did not respond, I would like to emphasize that Introduce binary-relation insead using pairs from $X\times Y$ directly is obviously unnecessary complication. This actually makes old definition essentialy more difficult.

Kamil Kielczewski (talk) 15:37, 23 February 2024 (UTC)

I see no references to published sources in your long posts in this thread. Are you proposing to replace the sourced definitions in our article with original research? That's a non-starter. —David Eppstein (talk) 18:18, 23 February 2024 (UTC)

No - because definition in "Formal definition" section in article is not sourced. Above proposition is also no original research. This is old concept of define function as some set. My proposition is to replace unclear, complex fuzzy old definition with simple, clear and complete definition. Kamil Kielczewski (talk) 18:30, 23 February 2024 (UTC)

"This is old concept of define function as some set": you must provide a source that defines a function as a set.

"complex fuzzy, simple, clear and complete"". This is your own opinion. It has no value for Wikipedia, if you do not provide a reliable source asserting the same.

"Set {X, Y, W} where W is subset of $X\times Y$": this is nonsensical since the sets {X, Y, W} and {Y, X, W} are equal, while $X\times Y\neq Y\times X.$ In other words with your definition, alleged to be clear, the domain and the codomain are confused. D.Lazard (talk) 20:46, 23 February 2024 (UTC)

1. "This is your own opinion. It has no value for Wikipedia, " this is again false statement. Actually I give arguments above - you ignore them. If that arguments are wrong - give counter argument. However I will provide proof that old definition is not complete and fuzzy - step by step:

• definition states: "A function with domain X and codomain Y is a binary relation R between X and Y"
• top part of definition defines what is binary relation: "A binary relation between two sets X and Y is a subset of the set of all ordered pairs... $X\times Y$" - this means that R is set (of pairs).
• Ok so acording to this definition function f is set R which looks as follows $f=R=\{(x,y):x\in X\}$ where $\forall x\in X,\exists y\in Y,\left(x,y\right)\in R$ and $(x,y)\in R\land (x,z)\in R\implies y=z$.
• but there is a problem - if only set $f=R$ is given then we can not deduce codomain Y (because is not guaranteed that all elements from Y will be used in binary relation R) e.g $f=\{(x,x^{2}):x\in \mathbb {R} \}$ is suriective or not? It depends of what is te codomain of f... . If e.g. $Y=\mathbb {R} ^{+}$ then it is suriective, if $Y=\mathbb {R}$ is not suriective. But this definition that f is R not provide such information about Y...
• so this definition that function f is set R is not comlete (we lose information about codomain when we define function in that way).
• however, if someone state that function is something more than R because it also has domain and codomain - then this is fuzzy part: is funtion is binary relation R or not (not= something more than R) ?

2. "you must provide ..." - As you can see, old definition is very poor and week - actually old definition is wrong and does not meet the standards of modern mathematics (which can be verified by everyone - by above proof). I hope above proof is clear - However, if you have any doubts in above proof, indicate where. My goal is to remove wrong definition by good definition.

3. "this is nonsensical since the sets {X, Y, W} and {Y, X, W} are equal, while $X\times Y\neq Y\times X$" - again you write false statement. If you read new definition, then it is clear that domain is denoted by X, codomain is denoted by Y and W is subset of $X\times Y$. The order of elements in set f={W,Y,X} doesn't matter. If I wrtie $A=\{X,Y\}$ and $B=X\times Y$ where $X=\mathbb {R}$ and $Y=\mathbb {N}$ you will also say that set A and B are nonesense?

4. If you have still doubts then I will expalin one example - how to determine domain and codomain for function defined as follows: $f=\{\mathbb {R} ^{+},\mathbb {R} ,\{(x,x^{2}):x\in \mathbb {R} ^{+}\}\}$: 1. You detect that W is $\{(x,x^{2}):x\in \mathbb {R} \}$ because there is no other set of pairs. 2. You detect that first element of each pair is positive real number, and all positive real numbers are used - also you see that $\mathbb {R} ^{+}$ is element of f - so $\mathbb {R} ^{+}$ must be domain. 3. So the last element of set f which is $\mathbb {R}$ must be codomain Y. It is clear that f is not surjective. So based on informations included in new definition of f we can write it using traditional notation $f\colon \mathbb {R} ^{+}\to \mathbb {R} ,f(x)=x^{2}$ (but remember that it is only notation - not definition) - so new definition of function f contains ALL informations about this function.

5. In the other hand, if we try to define abouve funtion $f\colon \mathbb {R} ^{+}\to \mathbb {R} ,f(x)=x^{2}$ (this is only notation - dont confuse it with definition) according to old definition - it will be following binary relation (which is set of pairs which old definition states): $f=\{(x,x^{2}):x\in \mathbb {R} ^{+}\}$. Analysing this set we can determine that domain is $\mathbb {R} ^{+}$ but we can not determine what is codomain Y since we know that binary relation not allways use all elements of codomain. If someone see f dedined using only old definition and say that f is suriective or even it is bijection, - he will make mistake. That person based only on old definition of f will be unable to rewrite it coretly using traditional notation. Kamil Kielczewski (talk) 22:06, 23 February 2024 (UTC)

You wrote:"so acording to this definition function f is set R which looks as follows

$f=R=\{(x,y):x\in X\}$ where $\forall x\in X,\exists y\in Y,\left(x,y\right)\in R$ and $(x,y)\in R\land (x,z)\in R\implies y=z$."

This a wrong rewrite of the definition. The correct translation of the definition in terms of formulas is: a function from X to Y is a set R such that

${\big (}R\subseteq \{(x,y):x\in X,y\in Y\}{\big )}\land {\big (}\forall x\in X,\exists y\in Y,(x,y)\in R{\big )}\land {\big (}(x,y)\in R\land (x,z)\in R\implies y=z{\big )}$

As you are discussing a definition that is not the one given in our article, everything that you wrote after the above quotation is possibly correct (this is not my opinion) but is not relevant to a discussion on the content of the article.

If you prefer you may replace "a function from X to Y is a set R such that" with "a function is a triple $(X,Y,R)$ of sets such that". Personally, I find that the latter formulation is unnecessary pedantry.

D.Lazard (talk) 15:23, 24 February 2024 (UTC)

(I will continue points/threads numeration from my previous commment)

6. "This a wrong rewrite of the definition" - this is at most a misunderstanding (I assume that formula after word "and" is "one statement" (inside brakcets), and I assume it is obvious of context of our discussion that R is subset of $X\times Y$ - My oversight - I should have written in a more precise manner.). Taking this into account, we have described the same set R. But this is not crucial and does not change the correctness of the further argument. The key point in that proof was that R (and f) in old definition are only SET OF PAIRS - this cause the problem witch old definition.

7. "As you are discussing a definition that is not the one given in our article" - this is a false statement. I am discussing the definition given in the article.

8. "everything that you wrote after the above quotation ... is not relevant to a discussion on the content of the article." - this is a false statement. I divided my last comment into 5 separate usually independent points - you cannot assume that if one point had an error, all of them are wrong or off-topic. If any of these points are incorrect, present your arguments - unless you agree with them.

9. "a function is a triple" - as I wrote earlier, your proposal to use triple (X,Y,R) is an ill-conceived, overly complicated structure. A function can be defined using only a simple set {X,Y,W} due to special structure of graph W (because we can always distinguish the graph W from the domain X and the codomain Y due to the axiom of regularity) . Maybe instead of R or W, we should use just G (graph) which is a more descriptive symbol. Look on example presented in point 4 in my last comment. If you think this is not true - show me a counterexample: a set consistent with the new definition that can be read in an ambiguous way - as two different functions written in traditional notation. And show me a function written in traditional notation that cannot be written as a set consistent with the new definition (in similar way I show you counter example in point 5 in my last comment for old definition)

10. Moreover, introducing the additional concept of "binary relation" is an unnecessary multiplication of entities. Explain to me - what does binary relation bring to the definition of a function besides complication? We can suffice with a set of pairs instead. A good definition should be as simple as possible. Kamil Kielczewski (talk) 19:59, 24 February 2024 (UTC)

Per WP:DISENGAGE, only the last point deserve to be answered. My answer to this last point is that Wikipedia is neither a textbook nor a collection of mathematical monographies. For details see WP:Here to build an encyclopedia and WP:What Wikipedia is not. D.Lazard (talk) 19:10, 25 February 2024 (UTC)

11. "Wikipedia is neither a textbook nor a collection of mathematical monographies" -

So this is your argument for using the clearly redundant and complicating definition of "binary relation" (instead the direct "set of pairs")? You provided two links - I didn't find any justification in either of them for using unnecessary complications in simple mathematical definitions. Explain this.

12. In light of the link you provided "What Wikipedia is not" it follows that the current incorrect definition cannot be replaced by your hastily invented, not fully thought-out (personal invention) concept of using the triplet (X,Y,R). Especially since there are simpler, known concepts, for example, {X,Y,W}.

13. (re 10) "only the last point deserve to be answered" does not constitute a substantive argument for point 9, i.e., your personal proposal to use the triplet f=(X,Y,R) (which is overly complicated structure) instead of f={X,Y,W} (which is simpler but sufficient structure).

14. The old incorrect formal definition should be completely removed and replaced with a modern, simple, correct definition. Kamil Kielczewski (talk) 20:43, 25 February 2024 (UTC)

@Kamil Kielczewski – have you tried to figure out what other sources aimed at a broad audience of non-specialists use as a definition for "function"? (for example, general-purpose encyclopedias, introductory textbooks, expository survey papers for scientists or historians of science, semi-technical popular math books, etc.). As a general matter (I haven't examined this in great detail), in my opinion this article should ideally give a high-level survey of the most common conceptions of function typically employed by various kinds of mathematicians, each phrased in the most accessible way practical and thus hopefully at least somewhat legible for e.g. high school students and laypeople. I think we should shy away from excessively formal or technical details of various concepts, and should try to focus on the broad idea and its basic implications rather than picking on the pedantic edge cases of specific definitions. YMMV. –jacobolus (t) 02:04, 26 February 2024 (UTC)

15. I don't entirely agree with you. Mathematics is a specific field where there is no room for compromise - if something is wrong (which can often be easily demonstrated by showing a counterexample), it should not be accepted as correct - and mislead others. At the beginning of this thread (at the top), I proposed replacing the existing incorrect formal definition of function with a correct one that is simpler than the old one

16. The most challenging aspect of the new definition is $\forall x\in X,\exists !y\in Y:(x,y)\in W$ (old definition also use quantifiers), but we can add an explanation in natural language on how to read the quantifiers: for every x in X, there exists exactly one y in Y such that the pair (x,y) belongs to W. ). The new definition, due to its simplicity, is easier to understand than the old one

17. I agree that publishing various good solutions in one article is ok. Kamil Kielczewski (talk) 06:25, 26 February 2024 (UTC)

Just a comment regarding "wrong definition". Note that, as long as a definition is properly written in the language in question and defines something, there is no such thing as "wrong definition". You can judge a definition regarding whether it is convenient or inconvenient to express the ideas and theorems that you want, but not as being wrong. What can be wrong are theorems that make use of a definition. It is my personal preference the definition of functions as morphisms in Set. If I have to define it in a class, I choose that one, but there is nothing wrong with the definition as a relation. I do enjoy, for example, the notation that can be used in the latter, and that Halmos enjoyed to use of writing $(a,b)\in f$. There are always tradeoffs. What is wrong is those Calculus textbooks that define functions as a relation and then go to talk about codomains and surjectivity. Thatwhichislearnt (talk) 16:26, 11 March 2024 (UTC)

@D.Lazard You did not address points 11, 12, 13 and 14 (from my earlier comments) - I understand that you agree with them. If not, then present substantive arguments. Kamil Kielczewski (talk) 09:41, 27 February 2024 (UTC)

I disagree with everything you wrote and I have absolutely no obligation to respond to any of your injunctions ("present substantive arguments"). As said above, I disengage from this disruptive discussion. Please, consider the first sentence of this post as my answer to all your future posts on this talk page. D.Lazard (talk) 11:06, 27 February 2024 (UTC)

Your disagreement is just your private opinion - and I'm asking for arguments.

So what we do with current wrong old formal definition of function? (what was shown above).

Unless you want to leave it and mislead people Kamil Kielczewski (talk) 12:27, 27 February 2024 (UTC)

@Kamil Kielczewski If you want to persist in this one, you should (1) distill your criticism down to the shortest length you can make it, and (2) try to find existing published sources discussing this rather than basing it on your own logical reasoning, and link us to those sources. –jacobolus (t) 15:41, 27 February 2024 (UTC)

Regarding "The new formal definition you suggest is incorrect, since in a set, the elements are not ordered" Think again. One of the three is a subset of the Cartesian product of the other three such that the projection to the first component gives which one is the domain. The axiom of regularity ensures that which of the three must be the relation is uniquely determined. This in turn determines which is the domain. There is also no problem if the domain is equal to the codomain, which gets expressed as a set of two elements, instead of three. Strictly speaking there is no problem. The only problem is the unnecessary complexity. Thatwhichislearnt (talk) 15:46, 11 March 2024 (UTC)

My two cents. I do agree with the objections about sourcing. Even in those sources that look at functions as morphism, they don't define them in this way. It is a fact that there are in current use two definitions of functions that do define two different concepts: The set-theoretic definition as relations, and the definition as morphisms in the category Set. The article should mention the existence of the two, and pick one to carry for most of the rest, for consistency. I do think that the definition as relation is probably the more appropriate, only because it is still the most quoted. It is my opinion that that will change overtime, as literature updates, and people that have preferences for the older concept die out, but Wikipedia will also change overtime, when that happens. I do agree that the wordy definition in the lede is ambiguous, and that it does not allow to determine/define the codomain given the function (as it is the case with the set-theoretic definition). The supposed "mathematical objections" as either a set of domain, codomain and relation, yes are not correct. Lazard just doesn't have proper understanding of set theory. Thatwhichislearnt (talk) 16:12, 11 March 2024 (UTC)

The sentence "the functions are the morphisms of the category of sets" is not a definition of functions; it is the definition of the morphisms of the category of sets, and do not give any information on functions. Nevertheless, there are several nonequivalent definitions of a function that are in common use. Two of them are already in the articles (partial functions and multivariate functions are called "functions" in many texts). Another one is the definition of a function in lambda calculus, which does not carry any concept of a domain and a codomain.

Maybe you understand set theory better than me, but there is another thing that I do not understand. This is what is "wordy and ambiguous" in the first paragraph" (this is the unique place in the lede where a function is defined): A function from a set X to a set Y assigns to each element of X exactly one element of Y. The set X is called the domain of the function and the set Y is called the codomain of the function. Nevertheless, a sentence near the end of the lede was ambiguous, because the lack of "Given its domain and its codomain" before "a function is uniquely represented ...". I have fixed this.

D.Lazard (talk) 17:15, 11 March 2024 (UTC)

I agree with this change. The main locus of the dispute here seems to be how we are given its domain and its codomain: are they required to be part of the set-theoretic representation of a function, in order to make the correspondence between functions and the sets that represent them bijective, or can they be taken as metadata separate from the representation itself? I think your wording is appropriately agnostic on this point. —David Eppstein (talk) 17:41, 11 March 2024 (UTC)

Being agnostic will just keep a strong flow of people who notice the ambiguity and try to fix it or complain about it. It is better to be explicit and precise, in the appropriate section (Formal definition) and explicitly pick a choice to follow most of the article. Note, that the choice that you are implying, with the codomain as "metadata" is not what is done in the set-theoretical definition of function as a relation. While I personally prefer "codomain as metadata", there is lot of literature that either doesn't do that, or doesn't realize that is not doing that. Thatwhichislearnt (talk) 19:16, 11 March 2024 (UTC)

No. It is not the job of Wikipedia to make a choice, in situations for which mainstream scholarship has not made a choice. See WP:BALANCE, part of one of Wikipedia's core policies on maintaining a neutral point of view. —David Eppstein (talk) 21:15, 12 March 2024 (UTC)

You are misreading what I said. "explicitly pick a choice to follow most of the article" means for most of the article. In the section on Formal Definition, both choices in current use must be explicitly stated. As opposed to the nonsense that is right now written in that section. Currently what is written, has the same structure as "A prime with hophops, is a natural number that is greater than 1 and not a product of smaller natural numbers", where "with hophops" is playing the same role as "with domain X and codomain Y". In other words, it is not a definition of anything. Thatwhichislearnt (talk) 21:55, 12 March 2024 (UTC)

And that is the main reason people keep coming over and over trying to fix it. Because what it is said in English, when its formalization is extracted, is a non-definition. It leaves the predicate "with Domain X and codomain Y" undefined. It is not "agnostic", nor educative, nor "balanced". It is just badly written. Thatwhichislearnt (talk) 22:00, 12 March 2024 (UTC)

I am not using "the functions are the morphisms of the category of sets" to define, but to refer to a definition, given that the variants do not have single name that I can use to call them over and over. The sentence in green, the one in the lede, while OK-ish for the beginning of an article in Wikipedia, is also not a definition of anything. What is "assigns", for example. Likewise, the first sentence in the section Definition uses "is an assignment". Again, OK-ish for that location, but also not a definition. What is "assignment"? My biggest problem is with the one that purports being the formal definition "A function with domain X and codomain Y is a binary relation R between X and Y that satisfies the two following conditions", specially when combined with the sentence further down that says "These conditions are exactly the formalization of the above definition of a function." The first, when it says what the function "is" immediately makes the Y un-recoverable from what the function is, a relation. Then the sentence "These conditions are exactly the formalization of the above definition of a function." is just false. The intended definitions in all the OK-ish sentences from before, are not exactly what the formal definition expresses.

I entirely understand the intension of the "A function with domain X and codomain Y". It is being used with the intension of serving as the set on which a comprehension is restricted, in restricted comprehension. The set $A$, in $B=\{x\in A\mid \phi (x)\}$. It just doesn't work, because it hasn't been defined yet what is being "with domain X and codomain Y". Then when the "is" comes in the sentence, it makes it even worse. I don't mean that the formal definition has to be entirely formal. It is fine if written in English, but the English must imitate the stricture of the axiom of comprehension.

One way to see it more clearly, compare with some other definition in which the restricted comprehension is written properly. For example prime numbers "A natural number (1, 2, 3, 4, 5, 6, etc.) is called a prime number (or a prime) if it is greater than 1 and cannot be written as the product of two smaller natural numbers.". The predicate of being larger than 1 and not a product of smaller natural numbers is restricted to the set of natural numbers. The set of natural numbers, has been previously defined. Some irrational real numbers satisfy the predicate, but we are not calling them prime, because the definition is only for some natural numbers. Note also, how given a prime number we can still say that it is a natural number.

Compare also with an improperly written definition that has the same structure, but uses some other words: "A prime number with hopshops is a natural number that is greater than 1 and cannot be written as the product of two smaller natural numbers". It hasn't been defined what "with hophops" means and one cannot recover it from the predicate that is after the "is". Thatwhichislearnt (talk) 18:17, 11 March 2024 (UTC)

I suggest to cease discussing, or to base future contributions on reliable sources (WP:RS). This is what David Eppstein proposed on 23 Feb already. - Jochen Burghardt (talk) 17:21, 27 February 2024 (UTC)

Partial functions

The above lengthy discussions revealed me that, in many contexts, "function" is used in place of "partial function". This may confuse readers. This is my reason for adding a subsection § Partial functions of the section § Definition. I have tried to explain this confusing terminology with examples. D.Lazard (talk) 15:25, 5 March 2024 (UTC)

1. I do not see how partial functions would explain or solve anything in the above two threads "Formal definition ..." (e.g., the problem of surjectivity). Please clarify this.

2. You did not provide any citations

3. The 'Partial functions' section can remain in the article. It constitutes a separate thread." Kamil Kielczewski (talk) 06:12, 6 March 2024 (UTC)

§ Partial functions is not a subsection of § Formal definition; it is a subsection of § Definition, at the same level as § Formal definition and § Multivariate functions. D.Lazard (talk) 11:31, 6 March 2024 (UTC)

Riemann Hypothesis and partial functions

The section Partial Functions which has no source, contains this paragraph:

"Similarly, a function of a complex variable is generally a partial function with a domain of definition included in the set $\mathbb {C}$ of the complex numbers. The difficulty of determining the domain of definition of a complex function is illustrated by the multiplicative inverse of the Gamma function: the determination of the domain of definition of the function $z\mapsto 1/\Gamma (z)$ is equivalent to the proof or disproof of one of the major open problems in mathematics, the Riemann hypothesis."

1. I think that it means the Riemann zeta function, instead of the Gamma Function.
2. Also the meaning of the phrase : "the determination of the domain of definition" of the function $z\mapsto 1/\zeta (z)$ is not clear to me and also then meaning of "equivalent to the proof or disproof of" the Riemann hypothesis.
   For example, does it mean that if we prove that the R.H. is false (by giving a counterexample), then we can (or cannot) "determine" the domain of definition of the function $z\mapsto 1/\zeta (z)$"?

--Cbigorgne (talk) 17:38, 13 March 2024 (UTC)

Thank you for pointing this. I'll fix this. for the second point, I'll simply replace "equivalent" with "more or less equivalent". I think that it is a good way for making clear without entering in technical details that it not a true equivalence. D.Lazard (talk) 19:19, 13 March 2024 (UTC)