User Najib Idrissi

51 votes

4 answers

3k views

Have there been (successful) attempts to use something other than spheres for homotopy groups?

asked Oct 5, 2015 at 13:19

21 votes

1 answer

3k views

When is there a ring structure on an abelian group $(A,+)$?

asked Jun 30, 2013 at 8:46

16 votes

1 answer

721 views

If $S^1 \hookrightarrow X$ induces an injection of $H_1$, then $X$ retracts onto $S^1$

asked May 11, 2013 at 12:12

15 votes

1 answer

2k views

Tangent bundle of a quotient by a proper action

asked Apr 14, 2013 at 9:39

14 votes

1 answer

2k views

Why is the recognition principle important?

asked Jun 28, 2012 at 10:06

14 votes

2 answers

1k views

Is $\mathbb{R}^n$ properly homotopy equivalent to $\mathbb{R}^m$ if $n \neq m$?

asked Jun 8, 2015 at 9:13

14 votes

2 answers

949 views

What are examples of parallelizable complex projective varieties?

asked Dec 9, 2015 at 13:12

9 votes

2 answers

336 views

Proving $\{P(a)\}$ (where $a$ is transcendental) is dense

asked May 28, 2011 at 8:53

8 votes

2 answers

752 views

Where to learn about model categories?

asked Nov 27, 2015 at 12:25

8 votes

1 answer

692 views

What's the intuition behind "representable morphisms"?

asked Apr 5, 2017 at 14:30

7 votes

0 answers

200 views

What are types of coalgebras that are more naturally described by cooperads?

asked Feb 11, 2016 at 14:31

6 votes

2 answers

1k views

Is the pairing induced by the wedge product and integration nondegenerate on de Rham forms?

asked Jan 18, 2016 at 14:44

6 votes

1 answer

406 views

What is this structure involving a monad and a comonad?

asked May 25, 2016 at 14:12

6 votes

2 answers

396 views

There is no "operad of fields"

asked Oct 12, 2013 at 14:11

5 votes

2 answers

1k views

What does it mean for a coalgebra to be cogenerated by a subspace?

asked Jul 27, 2013 at 9:04

5 votes

1 answer

2k views

The image of a proper scheme is closed

asked Dec 27, 2012 at 16:07

5 votes

0 answers

95 views

An interpretation of this construction giving an operad from a bialgebra?

asked Feb 24, 2016 at 13:57

4 votes

1 answer

446 views

Is the special orthogonal group really rationally homotopy commutative?

asked Mar 18, 2016 at 10:14

4 votes

1 answer

259 views

How to tell if a manifold can be embedded as the interior of a compact manifold with boundary?

asked May 19, 2016 at 13:06

4 votes

1 answer

493 views

How was the functorial factorization axiom "frequently misstated"?

asked Mar 7, 2015 at 9:11

3 votes

1 answer

72 views

Do "partial shuffles" have a name?

asked Nov 8, 2019 at 13:50

3 votes

1 answer

437 views

Why are left/right proper model categories called so?

asked Mar 31, 2016 at 12:18

2 votes

1 answer

396 views

How is this "Stiefel manifold"-like fiber bundle called? How to characterize it?

asked Jan 22, 2016 at 17:18

2 votes

1 answer

172 views

Is it really necessary to choose a basis to prove that every element of a comodule is contained in a finite-dimensional subcomodule?

asked Mar 30, 2016 at 9:32

2 votes

2 answers

1k views

How to understand cocategories

asked Apr 30, 2015 at 8:49

2 votes

1 answer

156 views

If a morphism of abelian groups is an isomorphism when tensorized with $\mathbb{R}$, is it also an isomorphism when tensorized with $\mathbb{Q}$?

asked Oct 6, 2015 at 7:35

2 votes

1 answer

732 views

Understanding whether a ramified covering ramifies over infinity

asked Oct 23, 2012 at 9:51

1 vote

2 answers

168 views

The image of a certain morphism in $\mathbb{A}^1_k$ is a closed point

asked Jan 1, 2013 at 18:32

1 vote

1 answer

91 views

Is it possible to cut the unit square in triangles of the same area such that one of the vertices has irrational coordinates? [duplicate]

asked Oct 29, 2018 at 20:54

0 votes

0 answers

116 views

How to call a "non-strict" monoidal category?

asked Apr 28, 2016 at 12:12

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31 Questions

Have there been (successful) attempts to use something other than spheres for homotopy groups?

When is there a ring structure on an abelian group $(A,+)$?

If $S^1 \hookrightarrow X$ induces an injection of $H_1$, then $X$ retracts onto $S^1$

Tangent bundle of a quotient by a proper action

Why is the recognition principle important?

Is $\mathbb{R}^n$ properly homotopy equivalent to $\mathbb{R}^m$ if $n \neq m$?

What are examples of parallelizable complex projective varieties?

Proving $\{P(a)\}$ (where $a$ is transcendental) is dense

Where to learn about model categories?

What's the intuition behind "representable morphisms"?

What are types of coalgebras that are more naturally described by cooperads?

Is the pairing induced by the wedge product and integration nondegenerate on de Rham forms?

What is this structure involving a monad and a comonad?

There is no "operad of fields"

What does it mean for a coalgebra to be cogenerated by a subspace?

The image of a proper scheme is closed

An interpretation of this construction giving an operad from a bialgebra?

Is the special orthogonal group really rationally homotopy commutative?

How to tell if a manifold can be embedded as the interior of a compact manifold with boundary?

How was the functorial factorization axiom "frequently misstated"?

Do "partial shuffles" have a name?

Why are left/right proper model categories called so?

How is this "Stiefel manifold"-like fiber bundle called? How to characterize it?

Is it really necessary to choose a basis to prove that every element of a comodule is contained in a finite-dimensional subcomodule?

How to understand cocategories

If a morphism of abelian groups is an isomorphism when tensorized with $\mathbb{R}$, is it also an isomorphism when tensorized with $\mathbb{Q}$?

Understanding whether a ramified covering ramifies over infinity

The image of a certain morphism in $\mathbb{A}^1_k$ is a closed point

Is it possible to cut the unit square in triangles of the same area such that one of the vertices has irrational coordinates? [duplicate]

How to call a "non-strict" monoidal category?