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53 votes

Bijection $f\colon\mathbb{N}\to\mathbb{N}$ with $f(0)=0$ and $|f(n)-f(n-1)|=n$

sequences-and-series combinatorics natural-numbers asked Jun 21, 2018 at 20:26
math.stackexchange.com

28 votes

Is $\sum_{a=0}^m\sum_{b=0}^n\cos(abx)$ always positive?

real-analysis inequality trigonometric-series asked Oct 18, 2021 at 2:54
math.stackexchange.com

23 votes

Abstract proof that $\lvert H^2(G,A)\rvert$ counts group extensions.

group-theory homological-algebra group-cohomology group-rings group-extensions asked Apr 30, 2019 at 22:01
math.stackexchange.com

22 votes

Why is the regulator of the degree 11 extension of $\mathbb{Q}$ so large?

nt.number-theory algebraic-number-theory zeta-functions iwasawa-theory cyclotomic-fields asked Mar 22, 2021 at 19:51
mathoverflow.net

21 votes

Probability of random sphere lying inside the unit ball

probability geometry spheres geometric-probability asked Feb 22, 2020 at 2:27
math.stackexchange.com

21 votes

Maximal subgroups that force solvability.

group-theory finite-groups solvable-groups asked Sep 21, 2018 at 5:48
math.stackexchange.com

18 votes

Fundamental group of punctured simply connected subset of $\mathbb{R}^2$

at.algebraic-topology gr.group-theory fundamental-group asked Dec 16, 2019 at 18:45
mathoverflow.net

14 votes

Is a product of $p$-subgroups still a $p$-subgroup?

group-theory p-groups asked Sep 1, 2021 at 17:28
math.stackexchange.com

13 votes

Group with fewer than $p^2$ Sylow p-subgroups

gr.group-theory finite-groups asked Oct 12, 2020 at 18:01
mathoverflow.net

13 votes

Fundamental group of punctured simply connected subset of $\mathbb{R}^2$

group-theory algebraic-topology fundamental-groups asked Jun 11, 2019 at 5:31
math.stackexchange.com

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