user avatar
user avatar
user avatar
Zhen Lin
  • Member for 11 years, 7 months
  • Last seen this week
101 votes
5 answers
10k views

What concept does an open set axiomatise?

86 votes
2 answers
12k views

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

67 votes
1 answer
6k views

Simplicial homology of real projective space by Mayer-Vietoris

43 votes
4 answers
7k views

Proving the snake lemma without a diagram chase

40 votes
2 answers
1k views

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

37 votes
1 answer
1k views

How much set theory does the category of sets remember?

36 votes
4 answers
5k views

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

35 votes
8 answers
8k views

Examples of surjective sheaf morphisms which are not surjective on sections

34 votes
1 answer
4k views

Does localisation commute with Hom for finitely-generated modules?

34 votes
5 answers
30k views

When does pointwise convergence imply uniform convergence?

33 votes
1 answer
15k views

Strong and weak convergence in $\ell^1$

33 votes
0 answers
835 views

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

28 votes
2 answers
2k views

What is the coproduct of fields, when it exists?

24 votes
1 answer
5k views

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

23 votes
1 answer
905 views

Model existence for infinitary logics

23 votes
2 answers
2k views

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

22 votes
1 answer
3k views

Pullbacks of categories

21 votes
1 answer
2k views

Tensor products of infinite-dimensional spaces and other objects

21 votes
2 answers
8k views

The Laplace transform of the first hitting time of Brownian motion

21 votes
1 answer
929 views

Non-axiomatisability and ultraproducts

21 votes
1 answer
2k views

The categories Set and Ens

19 votes
0 answers
635 views

Bousfield–Kan spectral sequence for homotopy colimits

16 votes
2 answers
2k views

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

16 votes
2 answers
2k views

The equivalence of two formulae for the Laplace—Beltrami operator

16 votes
1 answer
1k views

What are the algebras of the double powerset monad?

15 votes
5 answers
6k views

Angle brackets for tuples

15 votes
1 answer
646 views

What are the universally effective epimorphisms of topological spaces?

14 votes
2 answers
2k views

In what senses are archimedean places infinite?

14 votes
3 answers
859 views

What fragment of ZFC do we need to prove Zorn's lemma?

13 votes
2 answers
2k views

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