Talk:Radon measure

Equations are atrocious

The author has left a lot of work for others, as the mathematics is presented in LaTeX and needs to be formatted properly in order to be readable online.

Radon distance - a metric?

In the article, the Radon distance is defined as a supremum of integrals of bounded continuous functions. However, if the measure m1 is not finite and if the measure m2 is the zero measure, we could take f to be the constant function equal to 1 and then the distance of m1 and m2 is m1(X) which is plus infinity. This is not suitable for a metric! Therefore, this part of the article needs to be corrected (e.g. by considering only finite Radon measures). ASlateff 128.131.37.74 13:18, 11 June 2007 (UTC)

Integration

The section when Integration is defined is confusing. I do not understand what the vectorspace F has to do with anything. —Preceding unsigned comment added by MathHisSci (talk • contribs) 17:02, 24 January 2010 (UTC)

Recent edits clarify things, thank you. MathHisSci (talk) 21:34, 6 February 2010 (UTC)

Measures

In the section "Measures", there is some confusion about whether ${\mathcal {K}}(X)$ refers to the set of all continuous real-valued functions or only to those with compact support. The latter is probably true. In that case, it may be useful to point out that without positivity, Radon measures are not necessarily measures: I think that the mapping

$f\mapsto \int _{\mathbb {R} }f(x)\,\sin(x)\,dx$

is a (complex-valued) continuous linear functional on ${\mathcal {K}}(\mathbb {R} )$, which represents a signed measure on any compact subset of $\mathbb {R}$, but not on $\mathbb {R}$ itself because $\mu (\mathbb {R} )$ cannot be defined.--146.107.3.4 (talk) 09:42, 10 December 2007 (UTC)

You're right, ${\mathcal {K}}(X)$ refers to continuous functions with compact support on X. Thanks for attentiveness — I fixed that one now.

As for non-positive linear functionals and measures, it is a question of conflicting terminologies: Bourbaki and other proponents of the described approach to measure theory generally call measures what others call signed measures, while using the term positive measure for what others call simply a measure. I have clarified this in the introduction to the section in the article. Stca74 (talk) 12:23, 11 December 2007 (UTC)

error in Examples

Note for an expert in the field: In the Example section the second example of those which are not Radon measures is incomplete and is in error. Somebody needs to fix it. HowiAuckland (talk) 20:36, 5 August 2010 (UTC)

I think the correct definition is "any set which contains an uncountable closed subset of the open interval [1,omega)" (i.e. the final point is not required and has measure 0). I need to think, though, before correcting the article, if with this definition the claim is correct.--132.76.81.190 (talk) 15:01, 6 June 2019 (UTC)

The article falsely claimed that the space of probability measures was always compact under the Wasserstein metric. This is true only if the underlying space is also compact (counterexamples are trivial to construct); I just fixed that. As a general remark, there are not enough references in this article. Hairer (talk) 12:41, 2 October 2011 (UTC)

Direct limit of topological spaces vs. direct limit of locally convex topological vector spaces

Within the article, ${\mathcal {K}}(X)$ has been defined as a "direct limit of topological spaces ... equipped with the direct limit topology". Shouldn't ${\mathcal {K}}(X)$ be defined as a direct limit of locally convex topological vector spaces, carrying the coarsest locally convex topology making the injections continuous? Bourbaki's Topological Vector Spaces discusses this topology, which is called the final locally convex topology, and it does not appear to coincide with the final topology (i.e. the finest topology making the injections continuous). Is there some reason why the two topologies would coincide in this case? 174.91.40.227 (talk) 23:10, 16 May 2011 (UTC) 174.91.40.227 (talk) 23:14, 16 May 2011 (UTC)

You're right, locally convexity needs to be imposed. This can be fixed easily. For the base of the topology, instead of taking subsets whose preimages are open, take convex balanced subsets. I assume by "coarsest", you mean finest.

Incidentally, tt is really an overkill to be talking about LCTVS here. Much simpler, and with no cost, to take the Banach completion and simply consider the Banach space of continuous functions vanishing at infinity. Mct mht (talk) 15:00, 31 March 2013 (UTC)

Problem with Definition(s)

The section titled Definitions begins with this statement:

"Let m be a measure on the σ-algebra of Borel sets of a Hausdorff topological space X."

As long as this section is defining what a Radon measure is, it should do this carefully. But the word "measure" without qualification could mean a finitely-additive one or countably-additive one; one which takes values in the nonnegative reals, in the unrestricted reals, in the complexes, or in one of these number systems extended to include infinity.

Some intended properties of the "measure" here can be guessed from context . But this is an encyclopedia, not a guessing game. Exactly what kind of measure is intended here should be stated explicitly.

ALSO: The definition of "outer regular" may be quite important to include in this article (I don't know), but it should not be defined in the middle of the definition of Radon measure — since that definition does not depend on the definition of "outer regular". (The definition of "outer regular" can be placed after the definition of Radon measure, if it is needed.)Daqu (talk) 19:19, 16 October 2014 (UTC)

insufficient source cited by user:Limit-theorem

this seems to be all the rage by the so-called mathematicians in "mathematical finance" (the biggest joke there is).

i've demonstrated on previous occasions that the values purported to be "measurements" in financial mathematics are not measures at all.

this stems from the fact that no "asset" (stock, bond, whatever) can be modeled by the laws of nature as a function of time.

the fact that user:Limit-theorem is citing a financial mathematics book to "add" an unproven property is unacceptable.

the definition was fine before, and he seems to think that mathematical finance people know more than people specialising in mathematical analysis.

i am not disputing the validity or potential usefulness of this measure, but the addition by user:Limit-theorem adds nothing.

in fact, it's almost as if he's trying to deliberately downplay the importance and central aspect of the Lebesgue measure.

(Redacted)

i hate to keep bringing this up, but if renowned pure mathematician Sir Andrew Wiles has called out the field http://www.ibtimes.co.uk/andrew-wiles-maths-oxford-university-bankers-times-511492 (to which no sufficient answer has been provided), then that is telling about these individuals' abuse of mathematical tools.

STOP IT 174.3.155.181 (talk) 23:48, 26 April 2016 (UTC) edit: furthermore, THERE IS NO TOPOLOGY (which is REQUIRED) for which the application of measure theory to mathematical finance is valid. THERE IS NO GEOMETRY for the *exact* same reason that the measurements aren't continuous over time: the numbers being passed off as "natural" are contrived by human beings, and thus do not follow the laws of nature (no continuity)! abusing a beautifully abstract definition of measurability (which assumes continuity for the function being measured) to suit your dishonest conduct isn't acceptable!! — Preceding unsigned comment added by 174.3.155.181 (talk) 23:53, 26 April 2016 (UTC)

edit: ping user:Materialscientist user:Chjoaygame: this guy won't rebut the very clear argument that the measurable function has a geometry, which is then exploited by calculating the area with respect to the horizontal axis. we know that price movements are set by humans (nominal) and therefore are not real-valued (the representation may be, but the "value" reflected by the representation is not). 174.3.155.181 (talk) 17:02, 28 April 2016 (UTC)

(Redacted) 174.3.155.181 (talk) 17:13, 28 April 2016 (UTC)

Why is financial mathematics in the intro?

Regardless of what you think about the levels of rigor in financial mathematics (as in the argument directly above this), I am very skeptical of mentioning applications to financial mathematics in the head paragraph, particularly with that blurb as short as it currently is. --Dylan Thurston (talk) 18:51, 11 July 2016 (UTC)

Agree. Will remove. The intro requires some more sensible motivation that should be provided, though. Stca74 (talk) 12:02, 27 November 2016 (UTC)

Inner regularity is only required for open sets

A Radon measure is usually only required to be inner regular on open sets (rather than all measurable subsets). Otherwise, not all Haar measures are Radon. (An example is given here).

Is it okay if I edit this?

Conflicting definitions

The head section says that a Radon measure needs to be only inner regular, whereas the section Definitions requires both inner regular and outer regular. Clearly one or the other needs to change. I believe Folland requires both inner and outer regular, though I don't have the book with me right now. Minimal Counterexample (talk) 20:37, 10 May 2018 (UTC)

Reference problem

The section Radon measures on locally compact spaces contains two references that do not parse properly.

I hope someone can fix this.

There is no reason for an editor to leave the article in such a state, since failed parsings are quite obvious from their red text if the edit is previewed before it is published. 2601:200:C000:1A0:3932:3D49:97C7:A86A (talk) 19:06, 21 September 2021 (UTC)