Toby Bartels
  • Member for 11 years, 5 months
  • Last seen this week
What is the most useful non-existing object of your field?
19 votes

In intuitionistic mathematics, a non-constant function from $ \mathbb R $ to $ \{ 0 , 1 \} $. Many classical theorems can be proved to fail intuitionistically by showing that they imply this or ...

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Segal's Original Definition of a Topological Category
Accepted answer
16 votes

I would call this an internal category in the category of topological spaces and continuous maps.

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Why is a topology made up of 'open' sets?
15 votes

There are several interpretations of the original question, but one is, why focus on open sets rather than closed sets? I have an unusual answer. Suppose you want to do constructive mathematics. (Don'...

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Not quite adjoint functors
12 votes

Let $C$ and $D$ both be the category of finite-dimensional (say real) vector spaces and invertible linear maps between them, let $F$ be the identity, and let $G$ take a vector space to its dual. ($G$ ...

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Set theories without "junk" theorems?
10 votes

Although it's a little wordy, there is a method of formalizing things that avoids these theorems. To be sure, theorems such as $ \{ \{ \} , \{ \{ \} \} \} \in \{ \{ \} , \{ \{ \} , \{ \{ \} \} \} \} $...

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Interesting Applications of the Classical Stokes Theorem?
10 votes

In the theory of electromagnetism, the classical Stokes Theorem moves between the differential and integral forms of two of Maxwell's four equations; see https://en.wikipedia.org/wiki/Stokes%...

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Does equality between sets contradict the philosophy behind structural set theory?
9 votes

I can't give you an answer that fully addresses what Lawvere and Rosebrugh were thinking, since I haven't asked them. (If you want to ask them, Rosebrugh runs a category-theory mailing list at https:/...

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What are the occurrences of stacks outside algebraic geometry, differential geometry, and general topology?
8 votes

Mike Shulman has stack semantics, an application of stacks to logic. This is basically sheaf semantics, a now standard application to logic of sheaves (far from their own origin in geometry), except ...

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The formal p-adic numbers
8 votes

Localic completion is awesome and is the way to answer your question as asked (which requests a locale). However, a simpler answer for getting the correct (internal) set of $p$-adic numbers (the one ...

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Observables and dimensional analysis
8 votes

A dimensionful real-valued quantity takes values in a $1$-dimensional real vector space (a ‘line’) rather than in the space of real numbers as such. Given two such quantities taking values in the ...

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What is an ideal-supporting algebra?
Accepted answer
8 votes

It looks like the term ‘ideal-supporting algebra’ was written by me and survived slightly more than a decade on Wikipedia without being altered. (Well, somebody added a hyphen, a change that I agree ...

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Chomp! without the law of the excluded middle
6 votes

I don't think that there is anything interesting about this constructively, as long as we limit ourselves to finite sets in the strictest sense. The proof that a winning strategy exists gives us no ...

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Math blog directory
5 votes

There's this nLab page: http://www.ncatlab.org/nlab/show/math+blogs and some other lists that it links to.

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Is there a characterization of the topos of finite sets in the internal language?
Accepted answer
3 votes

Characterization is too strong; there are many models of $\operatorname{ETCS}$, so its axioms don't characterize $\operatorname{Set}$ either. But I'll interpret your question as asking how the axioms ...

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a dcpo seen as a category: when does a dcpo map induce a functor with an adjoint?
3 votes

An adjunction between posets is precisely a Galois connection. (Paul has characterized these in the case where the posets are both complete.) English Wikipedia; nLab.

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In the category of sets epimorphisms are surjective - Constructive Proof?
2 votes

I want to write down a proof that comes naturally, in a way. This proof assumes that you can form the quotient of a set modulo an equivalence relation, but does not require powersets. (So it works ...

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Observables and dimensional analysis
2 votes

Here is another, completely different answer. As Carlo indicated, one can use units in which Planck's constant is $1$. This is no arbitrary choice, but one dictated by fundamental physics. ...

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Is the generalized Baire space complete?
2 votes

As has already been remarked by Lasse Rempe-Gillen, you need to know what a Cauchy net is, say from a uniform structure. But since you want $\kappa$ to be discrete [edit: I had ‘complete’ here once] ...

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Using slides in math classroom
2 votes

Like Terry Tao, I find the transience of slides to be a problem. This is one reason why I stopped using slides as such and began using a single continuous-scroll page for each topic. I lecture from ...

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Why do we teach calculus students the derivative as a limit?
2 votes

I'm interested in the differentials-based approach advocated by Dray and Manogue at Bridging the Vector Calculus Gap. This is for multivariable calculus, but they do discuss the one-variable version (...

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Sober except not $T_0$?
Accepted answer
1 votes

Well, I've decided to go ahead and use ‘with enough points’. There are a lot of reasons to restrict to $T_0$ spaces, over and above reasons to restrict to sober spaces, and at least within that ...

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Leibnizian calculus textbook
1 votes

This approach is suggested by Tevian Dray and Corinne Manogue in their program of Bridging the Vector Calculus Gap. They focus on multivariable calculus and differential forms, but they discuss ...

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Uniform continuity and boundedness
0 votes

I'm afraid that I don't like your proposed proof. You derive a bound on $f(x)$, namely $\epsilon + f(y)$, but this is not fixed. Although you may choose any positive $\epsilon$ you wish (which then ...

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