MathOverflow Community Digest

Top new questions this week:

A mysterious paper of Stallings that was supposed to appear in the Annals

In Stallings's paper Stallings, John, Groups with infinite products, Bull. Amer. Math. Soc. 68 (1962), 388–389. he briefly discusses how to prove "several generalizations" of Brown's ...

reference-request gt.geometric-topology soft-question  
asked by Laura 17 votes

Are Erdős polynomials irreducible?

Define the Erdős polynomial to be $f_n(x):= \sum \limits_{0 \leq i,j \leq n}^{}{x^{ij}}$ (the name is motivated by http://oeis.org/A027424). For example for $n=5$, the polynomial is given by $x^{25}+...

nt.number-theory polynomials  
asked by Mare 16 votes

Why do people say Gödel's sentence is true when it is true in some models but false in others?

I am a beginner, so this question may be naive. Suppose we have a (sufficiently strong) consistent first order logic system. Gödel's first incompleteness theorem says there exists a Gödel sentence $g$ ...

lo.logic  
asked by CouldntLoginToMyPreviousAcc 15 votes
answered by Peter LeFanu Lumsdaine 29 votes

Reference request for a proof of the two-square Theorem

One can show (see below for a sketch of a proof) that every odd prime number $p$ can be written in exactly $(p+1)/2$ different ways as $$p=a\cdot b+c\cdot d$$ with $a,b,c,d\in\mathbb N$ satisfying $\...

reference-request prime-numbers involutions  
asked by Roland Bacher 14 votes

Height functions on $\mathcal{M}_g(\overline{\Bbb{Q}})$ defined via dessins d'enfants?

Belyi's theorem establishes a correspondence between smooth projective curves defined over number fields and the so called dessins d'enfants which are bipartite graphs embedded on an oriented surface ...

ag.algebraic-geometry reference-request algebraic-curves moduli-spaces heights  
asked by KhashF 14 votes
answered by Ariyan Javanpeykar 11 votes

Combinatorial description of the Mandelbrot set

I have a very naïve question: can one find anywhere a combinatorial description of the Mandelbrot set? Let me try to be a bit more precise: is it possible to encode each of its "bulbs" by ...

reference-request mandelbrot-set  
asked by Ilia Smilga 12 votes
answered by Carlo Beenakker 13 votes

Low-order symmetric group 2-generation: n=5,6,8

In a comment at the recent question What is the standard 2-generating set of the symmetric group good for?, it was remarked that the symmetric groups $S_n$ for $n\gt 2$, $n\neq 5,6,8$, can be ...

gr.group-theory finite-groups  
asked by David Roberts 11 votes
answered by Geoff Robinson 12 votes

Greatest hits from previous weeks:

Most interesting mathematics mistake?

Some mistakes in mathematics made by extremely smart and famous people can eventually lead to interesting developments and theorems, e.g. Poincaré's 3d sphere characterization or the search to prove ...

soft-question big-list gm.general-mathematics  
asked by Ilya Nikokoshev 210 votes
answered by Daniel Moskovich 216 votes

The most outrageous (or ridiculous) conjectures in mathematics

The purpose of this question is to collect the most outrageous (or ridiculous) conjectures in mathematics. An outrageous conjecture is qualified ONLY if: 1) It is most likely false (Being hopeless is ...

ho.history-overview big-list  
asked by Gil Kalai 185 votes
answered by Joel David Hamkins 117 votes

What are "perfectoid spaces"?

This talk is about a theory of "perfectoid spaces", which "compares objects in characteristic p with objects in characteristic 0". What are those spaces, where can one read about them? Edit: A bit ...

arithmetic-geometry reference-request nt.number-theory  
asked by Thomas Riepe 132 votes
answered by Peter Scholze 239 votes

Ways to prove the fundamental theorem of algebra

This seems to be a favorite question everywhere, including Princeton quals. How many ways are there? Please give a new way in each answer, and if possible give reference. I start by giving two: ...

cv.complex-variables reference-request big-list  
asked by Anweshi 191 votes
answered by Gian Maria Dall'Ara 150 votes

What are the most misleading alternate definitions in taught mathematics?

I suppose this question can be interpreted in two ways. It is often the case that two or more equivalent (but not necessarily semantically equivalent) definitions of the same idea/object are used in ...

soft-question big-list examples teaching definitions  
asked by QPeng 140 votes
answered by Joel David Hamkins 249 votes

Nonequivalent definitions in Mathematics

I would like to ask if anyone could share any specific experiences of discovering nonequivalent definitions in their field of mathematical research. By that I mean discovering that in different ...

big-list definitions  
asked by Angeliki Koutsoukou Argyraki 131 votes
answered by Gerry Myerson 159 votes

What's a great christmas present for someone with a PhD in Mathematics?

Christmas is just around the corner and I haven't bought all the gifts for my family yet ( yeah, 😢) My Dad has a PhD in Mathematics, he works in Graph theory and his thesis was about Quasiperiodic ...

soft-question  
asked by Antenor Briareo 91 votes
answered by Deane Yang 90 votes

Can you answer these questions?

Occupation times for two-state Markov processes

Consider a two-state Markov process in continuous time, with states labelled $A$ and $B$. The transition rates for going from state $A$ to $B$, and state $B$ to $A$ are $\alpha$ and $\beta$ ...

pr.probability stochastic-processes st.statistics markov-chains  
asked by StatisticalMechanic 1 vote

Motivic homotopy categories closed under subobjets and quotients

It is well known that the category $\mathbf{HI}_{\rm Nis}^{\rm tr}(k)$ of $\mathbb{A}^1$-local Nisnevich sheaves with transfers is closed under subobjects and quotients, from the highly nontrivial ...

ct.category-theory sheaf-theory motives  
asked by user197402 1 vote

Minimum size of a Diophantine equation detecting the emptiness of a recursive set

I have a program $P$ taking an integer as input and outputting a Boolean value. It runs in polynomial time in the length of the input. There necessarily exists a Diophantine equation that has a ...

nt.number-theory algorithms computational-complexity  
asked by meirs 1 vote
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