User Zhen Lin - Mathematics Stack Exchange

132 votes

5 answers

14k views

What concept does an open set axiomatise?

asked Apr 9, 2011 at 5:23

101 votes

2 answers

20k views

Sheaf cohomology: what is it and where can I learn it?

asked Jul 31, 2011 at 13:25

70 votes

1 answer

6k views

Simplicial homology of real projective space by Mayer-Vietoris

asked Mar 14, 2011 at 18:16

49 votes

4 answers

8k views

Is $\operatorname{height} \mathfrak{p} + \dim A / \mathfrak{p} = \dim A$ true?

asked Jul 3, 2011 at 4:13

48 votes

5 answers

10k views

Proving the snake lemma without a diagram chase

asked Oct 22, 2011 at 16:47

47 votes

5 answers

39k views

When does pointwise convergence imply uniform convergence?

asked May 20, 2011 at 10:59

44 votes

2 answers

23k views

Strong and weak convergence in $\ell^1$ (Schur's theorem)

asked Jun 1, 2011 at 17:54

44 votes

1 answer

1k views

Closed model categories in the sense of Quillen [1969] vs the modern sense

asked Feb 10, 2013 at 23:41

43 votes

3 answers

2k views

Why do universal $\delta$-functors annihilate injectives?

asked Mar 30, 2012 at 0:37

42 votes

9 answers

11k views

Examples of surjective sheaf morphisms which are not surjective on sections

asked Aug 18, 2011 at 13:47

40 votes

2 answers

1k views

How much set theory does the category of sets remember?

asked Oct 31, 2014 at 16:26

36 votes

1 answer

5k views

Does localisation commute with Hom for finitely-generated modules?

asked Oct 25, 2011 at 18:19

29 votes

2 answers

2k views

What is the coproduct of fields, when it exists?

asked May 16, 2012 at 20:55

27 votes

3 answers

3k views

The bijection between homogeneous prime ideals of $S_f$ and prime ideals of $(S_f)_0$

asked Jul 29, 2011 at 14:34

27 votes

1 answer

6k views

The cardinality of a countable union of countable sets, without the axiom of choice

asked Jan 3, 2011 at 16:32

25 votes

1 answer

3k views

The categories Set and Ens

asked Jun 18, 2011 at 10:33

25 votes

1 answer

5k views

Pullbacks of categories

asked Jul 8, 2012 at 15:46

23 votes

1 answer

3k views

Tensor products of infinite-dimensional spaces and other objects

asked Aug 13, 2011 at 10:04

23 votes

2 answers

11k views

The Laplace transform of the first hitting time of Brownian motion

asked Mar 10, 2011 at 21:00

23 votes

0 answers

988 views

Bousfield–Kan spectral sequence for homotopy colimits

asked Nov 11, 2014 at 16:28

23 votes

1 answer

1k views

Model existence for infinitary logics

asked Dec 9, 2012 at 13:44

22 votes

1 answer

2k views

What are the algebras of the double powerset monad?

asked Dec 28, 2011 at 17:43

20 votes

1 answer

1k views

Non-axiomatisability and ultraproducts

asked May 26, 2011 at 10:21

19 votes

5 answers

8k views

Angle brackets for tuples

asked Jun 19, 2011 at 16:50

18 votes

2 answers

3k views

The equivalence of two formulae for the Laplace—Beltrami operator

asked May 2, 2011 at 17:09

18 votes

2 answers

4k views

Why are there no non-trivial regular maps $\mathbb{P}^n \to \mathbb{P}^m$ when $n > m$?

asked Oct 16, 2011 at 15:58

17 votes

1 answer

3k views

The projective model structure on chain complexes

asked Nov 11, 2013 at 21:15

16 votes

2 answers

3k views

Is a divisor in the hyperplane class necessarily a hyperplane divisor?

asked May 6, 2013 at 22:29

16 votes

1 answer

1k views

What are the universally effective epimorphisms of topological spaces?

asked Jul 30, 2015 at 15:34

16 votes

2 answers

1k views

Coherence for symmetric monoidal categories

asked Jan 28, 2012 at 15:39

Stack Exchange Network

128 Questions

What concept does an open set axiomatise?

Sheaf cohomology: what is it and where can I learn it?

Simplicial homology of real projective space by Mayer-Vietoris

Is $\operatorname{height} \mathfrak{p} + \dim A / \mathfrak{p} = \dim A$ true?

Proving the snake lemma without a diagram chase

When does pointwise convergence imply uniform convergence?

Strong and weak convergence in $\ell^1$ (Schur's theorem)

Closed model categories in the sense of Quillen [1969] vs the modern sense

Why do universal $\delta$-functors annihilate injectives?

Examples of surjective sheaf morphisms which are not surjective on sections

How much set theory does the category of sets remember?

Does localisation commute with Hom for finitely-generated modules?

What is the coproduct of fields, when it exists?

The bijection between homogeneous prime ideals of $S_f$ and prime ideals of $(S_f)_0$

The cardinality of a countable union of countable sets, without the axiom of choice

The categories Set and Ens

Pullbacks of categories

Tensor products of infinite-dimensional spaces and other objects

The Laplace transform of the first hitting time of Brownian motion

Bousfield–Kan spectral sequence for homotopy colimits

Model existence for infinitary logics

What are the algebras of the double powerset monad?

Non-axiomatisability and ultraproducts

Angle brackets for tuples

The equivalence of two formulae for the Laplace—Beltrami operator

Why are there no non-trivial regular maps $\mathbb{P}^n \to \mathbb{P}^m$ when $n > m$?

The projective model structure on chain complexes

Is a divisor in the hyperplane class necessarily a hyperplane divisor?

What are the universally effective epimorphisms of topological spaces?

Coherence for symmetric monoidal categories