MathOverflow Community Digest

Top new questions this week:

How large can subspaces of $U \otimes V$ be that avoid any pure tensors?

The Question is simple, yet I have encountered it multiple times in my mathematical life without finding an obvious answer, so I've decided to post it here. Say $U, V$ are real vector spaces of ...

ag.algebraic-geometry linear-algebra projective-geometry tensor-products  
user avatar asked by Jan Nienhaus Score of 26
user avatar answered by Lazzaro Campeotti Score of 7

Expected number of upsets in a knockout tournament

$2^n$ players $P_1, \dots, P_{2^n}$, ordered in decreasing order of skill are placed uniformly at random at the leaves of a binary tree of depth $n$. They play a knockout tournament according to the ...

co.combinatorics pr.probability  
user avatar asked by Nate River Score of 13
user avatar answered by Will Sawin Score of 18

Dirichlet series that gives power of $\pi$ at positive even integer

Let $f(s)$ be a Dirichlet series with algebraic coefficients, and suppose that: it admits a meromorphic continuation to $\mathbb{C}$; there exists $d \in \mathbb{N}$ such that $f(2n) \in \...

nt.number-theory rt.representation-theory algebraic-number-theory dirichlet-series  
user avatar asked by pisco Score of 12

Identities of Bernoulli numbers from rank 2 simple Lie algebras

Denote $B_n$ as Bernoulli numbers, it is known that the following three identities hold (for $n\in \mathbb{N}$): $$\tag{A}\label{504268_A}\frac{(2n)!}{(4n+1)!} \frac{-B_{6n+2}}{6n+2}= \sum_{k=0}^{2n} \...

nt.number-theory co.combinatorics rt.representation-theory sequences-and-series recurrences  
user avatar asked by pisco Score of 11

Comparison between Faltings height and naive height of a curve

Let $C$ be a smooth curve of genus $g\ge 2$ over a number field $K$, and let $J$ be its Jacobian variety. Suppose we have an embedding $C\to \mathbb{P}^2$ so that $C$ is defined by a homogeneous ...

ag.algebraic-geometry nt.number-theory abelian-varieties jacobians heights  
user avatar asked by Lorenzo Andreaus Score of 10
user avatar answered by Joe Silverman Score of 7

The strength of representing open sets

Is the following (second-order) formula schema provable in ATR$_0$? Let $\varphi$ be an arithmetical formula satisfying For all $x, y\in \mathbb{R}$, we have that $x=_\mathbb{R}y$ implies $\varphi(x)...

lo.logic reverse-math  
user avatar asked by Sam Sanders Score of 9

Fundamental class from smooth atlas

Let $M$ be an orientable $n$-dimensional manifold with a smooth oriented atlas $A :=\{(U_\alpha,\Psi_\alpha)\}_{\alpha}$ so that every non-empty $U_\alpha\cap U_\beta$ is contractible. Then we can ...

at.algebraic-topology differential-topology smooth-manifolds homology  
user avatar asked by Emilia Score of 9
user avatar answered by Liviu Nicolaescu Score of 12

Greatest hits from previous weeks:

Spectral sequences every mathematician should know

Reading mathematical articles, I sometimes see how mathematicians pull out amazing spectral sequences seemingly at will. While many are built using standard techniques like exact couples or filtered ...

at.algebraic-topology homological-algebra big-list spectral-sequences  
user avatar asked by Vanya Vasilev Score of 28
user avatar answered by Dave Benson Score of 25

What are some reasonable-sounding statements that are independent of ZFC?

Every now and then, somebody will tell me about a question. When I start thinking about it, they say, "actually, it's undecidable in ZFC." For example, suppose $A$ is an abelian group such ...

independence-results examples set-theory lo.logic big-list  
user avatar asked by Anton Geraschenko Score of 313
user avatar answered by Joel David Hamkins Score of 404

Example of a good Zero Knowledge Proof

I am working on my zero knowledge proofs and I am looking for a good example of a real world proof of this type. An even better answer would be a Zero Knowledge Proof that shows the statement isn't ...

cryptography computational-complexity  
user avatar asked by George Score of 68
user avatar answered by Ryan O'Donnell Score of 172

Thinking and Explaining

How big a gap is there between how you think about mathematics and what you say to others? Do you say what you're thinking? Please give either personal examples of how your thoughts and words differ, ...

soft-question ho.history-overview big-list big-picture math-communication  
user avatar asked by Bill Thurston Score of 414
user avatar answered by Terry Tao Score of 199

Reading list for basic differential geometry?

I'd like to ask if people can point me towards good books or notes to learn some basic differential geometry. I work in representation theory mostly and have found that sometimes my background is ...

dg.differential-geometry reading-list  
user avatar asked by GMRA Score of 87
user avatar answered by Alon Amit Score of 31

Examples of common false beliefs in mathematics

The first thing to say is that this is not the same as the question about interesting mathematical mistakes. I am interested about the type of false beliefs that many intelligent people have while ...

big-list mathematics-education  
user avatar asked by gowers Score of 1098
user avatar answered by Tilman Score of 809

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  
user avatar asked by Angeliki Koutsoukou Argyraki Score of 153

Can you answer these questions?

Reference for freeness of the ring generated by roots of unity

The following fact is well-known, and not hard to prove, but I do not know an explicit reference. Let $R$ be the subring of complex numbers generated by all roots of unity. Then $R$ is free as an ...

reference-request ac.commutative-algebra fields  
user avatar asked by Aurélien Djament Score of 2

Energy estimates for a class of quasilinear hyperbolic systems

Set $$ \lambda _{1}<\cdots<\lambda _{p}<0=\lambda _{p+1}<\cdots<\lambda _{n}, $$ consider the following system: $$ \partial _{t}+\lambda _{i}\partial _{x}u_{i}=\sum _{j\neq i}\lambda _{...

ap.analysis-of-pdes differential-equations hyperbolic-pde  
user avatar asked by Mr.Treeeee Score of 1

Cyclic 2-subgroups of GL(n,Z_p) for n>1

Let $p$ be a prime, $p\neq 2$. Let $Q$ be a cyclic $2$-group and $P$ an elementary abelian $p$-group of rank $n$. Suppose that $Q$ acts faithfully on $P$ (so $Q$ is isomorphic to a subgroup of $GL(n,...

gr.group-theory finite-groups group-actions p-groups cyclic-groups  
user avatar asked by Alessandro Giorgi Score of 1
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