User Thomas Browning

55 votes

3 answers

2k views

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

asked Jun 21, 2018 at 20:26

28 votes

1 answer

823 views

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

asked Oct 18, 2021 at 2:54

24 votes

0 answers

561 views

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

asked Apr 30, 2019 at 22:01

23 votes

0 answers

646 views

Maximal subgroups that force solvability.

asked Sep 21, 2018 at 5:48

23 votes

1 answer

854 views

Probability of random sphere lying inside the unit ball

asked Feb 22, 2020 at 2:27

14 votes

7 answers

1k views

If $2^{2k}-x^2\bigm|2^{2k}-1$ then $x=1$ [duplicate]

asked Nov 12, 2020 at 0:02

14 votes

1 answer

415 views

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

asked Sep 1, 2021 at 17:28

13 votes

1 answer

498 views

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

asked Jun 11, 2019 at 5:31

11 votes

2 answers

805 views

When is the compact-open topology locally compact?

asked Feb 24, 2022 at 22:13

10 votes

1 answer

362 views

Is there a prime of the form 711...117?

asked May 24, 2021 at 23:37

10 votes

4 answers

712 views

If $y^2-x^2\bigm|2^ky-1$ and $2^k-1\bigm|y-1$ then $y=2^k$ and $x=1$

asked Apr 12, 2019 at 6:19

9 votes

1 answer

155 views

Is $S_6$ the derived subgroup of some group?

asked Dec 15, 2019 at 1:40

8 votes

1 answer

251 views

Growth of powers of symmetric subsets in a finite group

asked Oct 3, 2023 at 6:23

7 votes

3 answers

186 views

If $p^k|n!$ then $p!^k|n!$

asked Jun 6, 2018 at 4:50

5 votes

1 answer

214 views

Kernel of $G\ast H\to G\times H$ is free

asked Jan 26, 2019 at 0:42

5 votes

1 answer

276 views

For odd primes $p$, are finite groups with self-normalizing Sylow $p$-subgroups solvable?

asked Aug 29, 2018 at 16:20

5 votes

0 answers

115 views

Criterion for $PSL(2,q)$

asked Oct 3, 2019 at 16:57

5 votes

3 answers

411 views

Combinatorial Proof of Stirling Number Inequality

asked Sep 8, 2020 at 0:06

5 votes

2 answers

291 views

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

asked Oct 10, 2020 at 17:12

5 votes

1 answer

153 views

When are homomorphisms automatically continuous?

asked Aug 15, 2023 at 19:47

4 votes

3 answers

413 views

The Diophantine equation $x^5-2y^2=1$

asked Aug 6, 2021 at 21:34

4 votes

1 answer

233 views

Parametrization of the curve $x^{x^y}=y$

asked Sep 23, 2020 at 5:23

4 votes

0 answers

114 views

Pattern in Number of Conjugacy Classes of p-groups

asked Mar 21, 2020 at 12:25

4 votes

0 answers

646 views

Conjugacy classes of a quotient group

asked Oct 25, 2019 at 16:15

4 votes

0 answers

728 views

Function where some derivative vanishes at every point.

asked Dec 14, 2017 at 16:06

4 votes

1 answer

1k views

Free subgroups of $PSL(2,\mathbb{Z})$ of index 6

asked Jan 26, 2019 at 0:33

3 votes

1 answer

366 views

Quotient of Riemann surface by properly discontinuous group action

asked Sep 23, 2019 at 21:39

3 votes

0 answers

132 views

Symplectic Forms on Abelian Groups

asked Apr 9, 2020 at 5:02

3 votes

0 answers

208 views

Loops through zero in Arnold's proof of the insolvability of the quintic

asked Jul 20, 2020 at 16:28

3 votes

2 answers

143 views

Is there a tessellation with no corners?

asked Sep 1, 2020 at 19:21

Stack Exchange Network

44 Questions

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

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

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

Maximal subgroups that force solvability.

Probability of random sphere lying inside the unit ball

If $2^{2k}-x^2\bigm|2^{2k}-1$ then $x=1$ [duplicate]

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

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

When is the compact-open topology locally compact?

Is there a prime of the form 711...117?

If $y^2-x^2\bigm|2^ky-1$ and $2^k-1\bigm|y-1$ then $y=2^k$ and $x=1$

Is $S_6$ the derived subgroup of some group?

Growth of powers of symmetric subsets in a finite group

If $p^k|n!$ then $p!^k|n!$

Kernel of $G\ast H\to G\times H$ is free

For odd primes $p$, are finite groups with self-normalizing Sylow $p$-subgroups solvable?

Criterion for $PSL(2,q)$

Combinatorial Proof of Stirling Number Inequality

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

When are homomorphisms automatically continuous?

The Diophantine equation $x^5-2y^2=1$

Parametrization of the curve $x^{x^y}=y$

Pattern in Number of Conjugacy Classes of p-groups

Conjugacy classes of a quotient group

Function where some derivative vanishes at every point.

Free subgroups of $PSL(2,\mathbb{Z})$ of index 6

Quotient of Riemann surface by properly discontinuous group action

Symplectic Forms on Abelian Groups

Loops through zero in Arnold's proof of the insolvability of the quintic

Is there a tessellation with no corners?