Talk:Artin reciprocity

Connection to more basic material?

The ignorance of the above comment aside, it would be nice to see how this connects to the more familiar types of reciprocity laws. As I am not a number theorist, I have no idea how involved such an explanation would be. At any rate, this article needs more, so I'll probably tag it when I figure out which tag is the right one to stick on here. VectorPosse 11:02, 3 March 2007 (UTC)

Okay, so I added an expert tag. It's not that I don't believe the material presented is correct; I just think an expert is required to add good material. VectorPosse 11:18, 3 March 2007 (UTC)

Yes, this is not the way to present Artin reciprocity! This "one version of the theorem" may be very useful in Langlands program, but it is neither a standard form of Artin's reciprocity law, nor is it useful for comprehension. I am a bit rusty on my global class field theory, but if no one fixes it, I will (eventually). Arcfrk 11:11, 10 March 2007 (UTC)

To answer VectorPosse's question above, the standard way of showing power reciprocity laws from the Artin reciprocity law requires introducing Hilbert symbols and computing them (this is done in section VIII.3 of Milne's Class Field Theory notes referenced in the article). As for the presentation of the Artin reciprocity law in this article, though not necessarily useful comprehension, this is probably the "cleanest" way to state it, and is definitely an extremely relevant and widely used interpretation of the theorem. I do definitely think a more classical statement should be given as well. RobHar 08:28, 24 March 2007 (UTC)

You mathematicians may scoff at the top comment but it actually makes a serious point. This and too many other mathematical wikipedia articles make absolutely no attempt to explain the topic in a way that is at least half understandable to the masses. Maybe you think this is the kind of topic that that only an expert can begin to comprehend but I think that is a failure of vision on your part. There have been many wonderful books written for general audiences about advanced mathematics (e.g. books on how Fermat's Last Theorem was proven and the search for a proof of the Riemmann Hypothesis) that have not shied away from at least giving a glimpse of some of the techniques involved. The way this page dives straight into highly technical terminlogy and equations presents a virtually infinite barrier to entry for the uninitiated. You guys get a D- in my book--Julian Brown (talk) 22:10, 5 June 2008 (UTC).

As someone who has contributed to this article I would just like to say that the place where you're comment is wrong, Julian, is in assuming that we, the mathematicians, are content with this article. I would love to make this article more understandable, and really say something nice and relevant, but that is unfortunately a very hard thing to do. I have not come across any account of Artin reciprocity that attempts to make its essence understood to the masses (and frankly in some sense it should only be upon finding such a reference that one should add anything to this wikipedia article, since wikipedia is an encyclopedia and should record what is known, not be a primary source). Many of the articles on higher-level mathematics are edited by an extremely small group of people attempting to put the bare bones of the idea in the article, and do not have the kind of community that may allow other, more popular, wikipedia articles to grow in comprehensiveness and comprehensibility. A project such as Wikipedia:Mathematics Collaboration of the Month attempts to create focussed communities to improve targeted math articles, perhaps an attempt to increase interest in such a project would aide in attaining the goal you seek.RobHar (talk) 01:50, 6 June 2008 (UTC)

And beyond RobHar's well-stated point, there is a degree to which advanced articles should be allowed to be advanced. I am a trained mathematician, but I don't pretend that I should be able to understand this article, even when it's well-written and entirely fleshed-out. Class field theory is complicated stuff and people spend years trying to understand even its basics. It's a bit presumptive to come here and pretend like anyone from off the street should be able to read this article once and understand it. I don't think this is an elitist way to think about it. Math is hard sometimes. The most we can hope for is that the lede is sufficiently "blue" that one could work their way back through the links and try to get some context for understanding this necessarily specialized article. VectorPosse (talk) 03:20, 6 June 2008 (UTC)

I don't expect the whole article to be easily accessible. But I think every entry in Wikipedia should start off with at least of couple of sentences (ideally a paragraph or two) that are intelligible to non-experts. I'm a firm believer in the idea that you should gradually walk your readers into deeper waters. This entry makes absolutely no effort to do that and as such is almost useless for someone who is not already steeped in the subject. I appreciate it might be hard to do in this case but I doubt very much that it is not possible. I wish I knew enough about the subject to make an attempt myself but I came here after talking with Ed Witten who said that he was working on the Geometric Langlands Program. I was hoping to get an insight into this subject but every associated entry I have read in Wikipedia is like reading hieroglyphics without a Rosetta Stone. --Julian Brown (talk) 07:41, 6 June 2008 (UTC)

I agree with what you're saying and the intro should definitely (and likely could) be made more accessible. This touches on one of the other points I made above, that there simply aren't very many editors around. In terms of how important Artin reciprocity is as an achievement in number theory, this article is ridiculously small and incomplete, but is actually better than the article on algebraic number theory itself, and if there were more editors around I think it would create more excitement about writing these articles. There are tonnes of articles I want to fix up, but time is not something I have a lot of. I've made some very minor edits to try to make it clearer as to why Artin reciprocity is related to Langlands. Geometric Langlands is a ridiculously complicated thing to set up, the only thing I'd feel comfortable saying on the subject is that in the classial Langlands correspondence one deals (approximately) with Galois representations, and Galois groups are etale fundamental groups and under an analogue of the Riemann-Hilbert correspondence representations of the etale fundamental group correspond to vector bundles with flat connection; as for the automorphic side, there I Hecke eigensheaves and I have no idea what those are. Here's something written by one of the leaders in the field that has some stuff on Langlands and geometric langlands that should have some understandable parts [1]. RobHar (talk) 09:53, 6 June 2008 (UTC)

Statement of the Artin reciprocity law

I don't have Neukirch at hand, so I can't verify whether he indeed calls the main isomorphism of the global class field theory "the Artin reciprocity law". Nonetheless, I do not think that it is a "standard" form of the reciprocity law: Hasse's and Tate's chapters in Cassels and Frölich, Weil's "Basic number theory" and I couple of other sources I was able to consult all give different versions of the reciprocity law, none of which involves the abelianization of the Galois group (or the Galois group itself, for the case of a finite abelian extension). It's difficult enough to discern why Artin's theorem is an example of a reciprocity law in the sense of algebraic number theory; stirring in a cohomological statement from global class field theory makes the confusion complete. With all due respect to the importance of precise statements of the results, perhaps, this is the exceptional case where examples and explanations of the meaning and significance should replace the statement?Arcfrk (talk) 21:21, 16 August 2008 (UTC)

I don't really have any access to references right now, but I'm pretty sure the current statement is pretty standard. The reciprocity map is a map from the ideal group (or ideles) to the Galois group, and the reciprocity law is a theorem about it, so I'm not even sure what it means to have a statement about it that doesn't involve a Galois group. Is it just something about congruences? Anyways, the references you listed were at least 40 years old, perhaps the statement of the law has been strealined in more recent texts, it could be interesting to describe this (possible) evolution.

As for the cohomological definition, it is certainly a very opaque one. It might be better to describe this map as taking a prime ideal and sending it to the Frobenius element of the Galois group. Starting with the finite abelian case is probably a good idea, too. However, I do think that the kind of description that is currently there should be present in some form, since it is an important approach to class field theory, but I think it should definitely be part of a later section. In fact, it is probably the standard approach in a class on class field theory, being more conceptual than the "less advanced" more ad hoc proofs, and involving more standard and important materal than Weil's "Basic number theory" approach through the theory of simple algebras.

In summary, what I'm saying is that I think that what is there should stay (perhaps in some improved form, and definitely as a later section in the article), but that I agree with you that examples, explanations, and less opaque statements should come first. RobHar (talk) 17:26, 17 August 2008 (UTC)

Well, perhaps I could say that I'm building up to it, if a little slowly. Modulus (algebraic number theory) was the first step. Richard Pinch (talk) 21:32, 18 August 2008 (UTC)

I would say it is more than a little slowly, since here it is 2009 and I still don't see that build up:(

But more to the point: the current statements of the Artin Reciprocity Law in this article do not even look like a reciprocity law. There is a reason, Cohen and Weyl, in their texts introducing this law, chose a notation that looked more like the classical reciprocity laws, and I think that reason should be respected in this article.

Unfortunately, as I write this, I cannot find my copy of Cohen, I can only find my copy of Weyl's "Algebraic Theory of Numbers", which classic though it is, uses notation that was not even standard then. But I will include it as an example of the kind of thing I am talking about, though I do not hope for his exact notation to become part of the article. Especially not his annoying overuse of Gothic letters!

Weyl defines the Artin symbol in notation very similar to that used for the Jacobi or Legendre symbols. Since I don't know Wiki's math notation well, I will reproduce it as closely as possible in Ascii. The Artin symbol is then written (k/y) where k is an Abelian field over K (the base field) and y is the prime ideal in k whose factors in K are the conjugates. The symbol (k/y) itself stands for the Frobenius substitution of the Galois group. This definition is then extended in the obvious way to fractional ideals, then in a less obvious way to ideles.

The reciprocity law then becomes (a/K)= 1 for any principal element a of J[k], where J[k] is defined as the group of all ideles with y-adic component non-zero, and a 'principal element' is a number in the multiplicative group of the base field. (op. cit p215 ch. IV sec. 16).

This is still not so accessible to the man on the street, but it is could be made accessible to a much wider audience than any definition that relies on cohomology, or on notation so remote from the historical notation for the quadratic reciprocity law.

68.164.80.179 (talk) 12:07, 13 April 2009 (UTC)

What would already be helpful (at least for mathematicians which are not number theorists) is adding an extra line that reminds us of the meaning of the symbol N(L/K) and other ones involved. Octonion (talk) 19:59, 22 February 2010 (UTC)

The example isn't really an example.

In the current version of the page, there is an "example" which consists of stating that the Artin map for a quadratic field is given by the Legendre symbol. While this is an example of an Artin map, I think it fails to be an example of the Artin reciprocity law, and it definitely fails to be an example of any connection to quadratic reciprocity.

I think finishing up this example with details of the connection to quadratic reciprocity would help the article greatly, especially in regards to providing a "connection to more basic material".

I don't feel comfortable doing it, since I'm still going through the process of reverse-engineering the connection myself. Hurkyl (talk) 05:02, 4 February 2012 (UTC)

Artin reciprocity for quadratic fields doesn't give you quadratic reciprocity. You need to go up to a cyclomotic field. If I have some time, I'll add in the example of the Artin map for a cyclotomic field and its relation to quadratic reciprocity. RobHar (talk) 16:43, 4 February 2012 (UTC)

 Done RobHar (talk) 06:58, 5 February 2012 (UTC)

Ah! That is very helpful, a much-needed addition to the page, thank you. Hurkyl (talk) 17:46, 8 February 2012 (UTC)

Requested move 8 December 2023

The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review after discussing it on the closer's talk page. No further edits should be made to this discussion.

The result of the move request was: Moved, unopposed (non-admin closure) BegbertBiggs (talk) 23:01, 22 December 2023 (UTC)

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Artin reciprocity law → Artin reciprocity – Like Eisenstein reciprocity, that page is not called “Eisenstein reciprocity law”, also suggest to add redirects: Quartic residue -> Quartic reciprocity, Octic residue -> Octic reciprocity (just like Cubic residue -> Cubic reciprocity) —— 2402:7500:903:48D1:E4B3:7991:B78C:3621 (talk) 05:54, 8 December 2023 (UTC) — Relisting. Bensci54 (talk) 13:48, 15 December 2023 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Define the symbols

The statement of the theorem should define the main symbols used, or at least point to definitions.

Let $L/K$ be a Galois extension of global fields and $C_{L}$ stand for the idèle class group of $L$. One of the statements of the Artin reciprocity law is that there is a canonical isomorphism called the global symbol map

$\theta :C_{K}/{N_{L/K}(C_{L})}\to \operatorname {Gal} (L/K)^{\text{ab}},$

where ${\text{ab}}$ denotes the abelianization of a group.

The main undefined things here are $N_{L/K}(C_{L})$ and $\operatorname {Gal} (L/K)$. I know what the second one means, so I'll add the definition of that. But I don't know the definition of the first, so someone should add that.

The map $\theta$ is defined by assembling the maps called the local Artin symbol, the local reciprocity map or the norm residue symbol

$\theta _{v}:K_{v}^{\times }/N_{L_{v}/K_{v}}(L_{v}^{\times })\to G^{\text{ab}},$

for different places $v$ of $K$.

Here none of the symbols $K_{v}^{\times }$, $N_{L_{v}/K_{v}}(L_{v}^{\times })$, or $G$ are defined, and I can't add the definitions myself. I can at least add a link that defines "place".

References