User HenrikRüping

21 votes

Example of an unnatural isomorphism

answered Aug 14, 2013 at 9:01

21 votes

Bijection $\varphi:\mathbb{N}\to\mathbb{N}$ that distorts every finite arithmetic progression

answered Jun 4 at 10:54

19 votes

Proving that a group is free

answered Mar 16, 2010 at 22:15

17 votes

Are there space filling curves for the Hilbert cube?

answered Aug 11, 2010 at 14:54

17 votes

The free group $F_2$ has index 12 in SL(2,$\mathbb{Z}$)

answered Oct 26, 2010 at 21:52

14 votes

Examples of common false beliefs in mathematics

answered Mar 4, 2016 at 3:54

12 votes

Results in linear algebra that depend on the choice of field

answered Sep 25, 2021 at 15:20

11 votes

Accepted

Isomorphic morphisms. A 27-morphism category

answered Sep 28, 2021 at 5:44

11 votes

Accepted

Homeomorphism-fixing subsets

answered May 22, 2018 at 14:01

8 votes

Accepted

Which functors preserve the number of connected components?

answered Oct 29, 2022 at 19:20

8 votes

What is an explicit example of a sequence converging to two different points?

answered Feb 19, 2010 at 10:14

8 votes

Accepted

homotopy associative $H$-space and $coH$-space

answered Feb 28, 2010 at 19:04

7 votes

Integrability of derivatives

answered Mar 5, 2010 at 11:48

7 votes

Applications of homotopy groups of spheres

answered Feb 26, 2010 at 10:46

7 votes

Accepted

Question about spectral sequences associated to filtered complexes with unbounded filtrations

answered Apr 5, 2019 at 7:33

7 votes

Visualizing hyperbolic metric of punctured sphere

answered Jan 21, 2020 at 21:17

7 votes

When is a extension of $\mathbb{Z}$ by a free group a CAT(0) group?

answered Dec 5, 2013 at 15:57

7 votes

Accepted

Criteria for extending vector field on sphere to ball

answered Feb 12 at 10:05

7 votes

Accepted

Equivariant Smith normal form?

answered Jul 1 at 10:16

7 votes

Accepted

A mutation of the Collatz disease

answered Jun 30, 2023 at 6:28

7 votes

Accepted

Reference for surjectivity of the canonical map $R^{G_1} \otimes R^{G_2} \to R^{G_1 \cap G_2}$

answered Aug 13 at 6:52

6 votes

For $\mathbb R^n \times Q \cong \mathbb R^m \times Q $ must $n = m$? ($Q$ is the Hilbert cube)

answered Nov 14 at 14:11

6 votes

When are two semidirect products of two cyclic groups isomorphic

answered Sep 27, 2023 at 18:13

6 votes

What is the intuitive difference between these two simplicial subdivision functors?

answered Oct 17, 2023 at 13:49

6 votes

Examples of non-metrizable spaces

answered Jan 14, 2011 at 12:49

6 votes

Accepted

The fundamental group of an $S^1$-quotient

answered Jul 23, 2013 at 12:33

6 votes

Accepted

Massey Products on a specific space

answered Oct 9, 2017 at 19:58

6 votes

Accepted

An example of a regular but not linear-based topological space

answered Jun 3, 2016 at 11:03

5 votes

Obstructions to $E_2$-algebra structure on $E_1$-algebra

answered Apr 23, 2019 at 17:12

5 votes

Adem relations of Steenrod square without modding out the coboundaries

answered Jul 12, 2018 at 14:41

Stack Exchange Network

93 Answers

Example of an unnatural isomorphism

Bijection $\varphi:\mathbb{N}\to\mathbb{N}$ that distorts every finite arithmetic progression

Proving that a group is free

Are there space filling curves for the Hilbert cube?

The free group $F_2$ has index 12 in SL(2,$\mathbb{Z}$)

Examples of common false beliefs in mathematics

Results in linear algebra that depend on the choice of field

Isomorphic morphisms. A 27-morphism category

Homeomorphism-fixing subsets

Which functors preserve the number of connected components?

What is an explicit example of a sequence converging to two different points?

homotopy associative $H$-space and $coH$-space

Integrability of derivatives

Applications of homotopy groups of spheres

Question about spectral sequences associated to filtered complexes with unbounded filtrations

Visualizing hyperbolic metric of punctured sphere

When is a extension of $\mathbb{Z}$ by a free group a CAT(0) group?

Criteria for extending vector field on sphere to ball

Equivariant Smith normal form?

A mutation of the Collatz disease

Reference for surjectivity of the canonical map $R^{G_1} \otimes R^{G_2} \to R^{G_1 \cap G_2}$

For $\mathbb R^n \times Q \cong \mathbb R^m \times Q $ must $n = m$? ($Q$ is the Hilbert cube)

When are two semidirect products of two cyclic groups isomorphic

What is the intuitive difference between these two simplicial subdivision functors?

Examples of non-metrizable spaces

The fundamental group of an $S^1$-quotient

Massey Products on a specific space

An example of a regular but not linear-based topological space

Obstructions to $E_2$-algebra structure on $E_1$-algebra

Adem relations of Steenrod square without modding out the coboundaries