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This tag is used if a reference is needed in a paper or textbook on a specific result.

21 votes
4 answers
4k views

Complete (possibly official) list of "What is..." articles from the Notices of the AMS

Does it exist online and where can one find it? (For example, these two sources are not official; is the longer one complete?)
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1 vote

On Ring Schemes

This is not a complete reference of course, but there is Lecture 26 (Ring Schemes; The Witt Scheme) in Mumford's book Lectures on curves on an algebraic surface
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3 votes

Measures of entangledness of an open curve

Perhaps, one could define the "entangledness" of an open curve (of unit lenghth and parametrized in arclength) as the minimum $$\mathrm{min}_{H}\;E(H), $$ over all possible smooth simple "strighten …
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1 vote
1 answer
184 views

What's the name of "twisted semidirect products"?

Let $V$ be an $n$-dimensional real vector space, $\Lambda\subseteq V$ a lattice, and $K$ a subgroup of $Aut_{\mathbb{Z}}(\Lambda)\cong GL(n,\mathbb{Z})$. Let also $\sigma \in Z^1(K,V/\Lambda)$, $\sigm …
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12 votes

Maxwell's equations and differential forms

M. Nakahara, Geometry, topology and physics. Paragraph 10.5 "Gauge theories", specifically 10.5.1 "$U(1)$ gauge theories". R.S. Palais, The geometrization of physics, lecture notes from a course at N …
47 votes
11 answers
15k views

Standard model of particle physics for mathematicians

If a mathematician who doesn't know much about the physicist's jargon and conventions had the curiosity to learn how the so called Standard Model (of particle physics, including SUSY) works, where sho …
11 votes
1 answer
2k views

Do complete non-projective varieties arise "in nature"?

I'm aware of the existence of complete (abstract) algebraic varieties that are not projective but, probably due to my ignorance, I have the impression that they arise only as very particular examples …
19 votes
1 answer
1k views

A result on Lie group actions on 15-dimensional spheres?

In this interview by Eric Weinstein to Roger Penrose, Timestamp 1:24:05., what result is the host talking about? Transcription of the relevant part: "If you have two sets of symmetries, known as Lie …
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3 votes
1 answer
316 views

Equivariant characteristic classes on $\mathbb{P}^n$

Let $T=(\mathbb{C}^*)^n$ act on $\mathbb{P}^n$ torically by $$t.[x_0:\dots:x_n]=[x_0\;:\;t_1x_1\;:\;\ldots \;:\;t_nx_n]$$ I would like to know an expression for the equivariant Chern character $\ma …
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4 votes
2 answers
382 views

About the cone being unique up to non-unique isomorphism

In an answer to this MO question [link] Fernando Muro sais: the mapping cone of a morphism in a triangulated category is unique up to non-unique isomorphism. This fact has originated a lot of re …
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1 vote

The symmetry group of $\mathbb Z^d$

As far as I read (See page 138 of R.W.Sharpe), the Erlangen program, strictly speaking, describes connected homogeneus manifolds $X$ as $G/H$ where $G$ is a Lie group considered as the "automorphism g …
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3 votes
3 answers
472 views

Undecidability and holomorphic functions (Reference request)

The goal of this question is to recall a certain mathematical fact -not in my field- that I was once briefly told and that I have fogotten, and also to collect similar results. The fact, I think, wa …
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6 votes
4 answers
2k views

Isomorphic but non-conjugate subgroups of $GL(n,\mathbb{Z})$ ?

I've been asked some questions by a friend interested in crystallography, and the following questions (I'm not an expert) came spontaneous to me: 1) Are there two finite subgroups $P,P'\subset\mat …
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21 votes
2 answers
858 views

Do Betti numbers beyond the first have a "number of cuts" interpretation?

I have heard stated the following Theorem. If $\Sigma$ is a (orientable) surface, then $\mathrm b_1(\Sigma)$ counts the maximum number of "circular cuts" (embedded circles $C_1,\ldots,C_m$) that you …
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9 votes
3 answers
2k views

Definition of étale (etc) for non-representable morphisms of algebraic stacks?

I've stumbled upon the statement that the morphism $\pi$ from a root stack of the form $\sqrt[r]{\mathscr{L}/\mathscr{Y}}$ (i.e. the "generic" version, not the one concentrated along a divisor) to its …
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