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Zhen Lin's user avatar
Zhen Lin's user avatar
Zhen Lin
  • Member for 15 years, 4 months
  • Last seen this week
132 votes
5 answers
14k views

What concept does an open set axiomatise?

101 votes
2 answers
20k views

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

70 votes
1 answer
6k views

Simplicial homology of real projective space by Mayer-Vietoris

49 votes
4 answers
8k views

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

48 votes
5 answers
10k views

Proving the snake lemma without a diagram chase

47 votes
5 answers
39k views

When does pointwise convergence imply uniform convergence?

44 votes
2 answers
23k views

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

44 votes
1 answer
1k views

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

43 votes
3 answers
2k views

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

42 votes
9 answers
11k views

Examples of surjective sheaf morphisms which are not surjective on sections

40 votes
2 answers
1k views

How much set theory does the category of sets remember?

36 votes
1 answer
5k views

Does localisation commute with Hom for finitely-generated modules?

29 votes
2 answers
2k views

What is the coproduct of fields, when it exists?

27 votes
3 answers
3k views

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

27 votes
1 answer
6k views

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

25 votes
1 answer
3k views

The categories Set and Ens

25 votes
1 answer
5k views

Pullbacks of categories

23 votes
1 answer
3k views

Tensor products of infinite-dimensional spaces and other objects

23 votes
2 answers
11k views

The Laplace transform of the first hitting time of Brownian motion

23 votes
0 answers
988 views

Bousfield–Kan spectral sequence for homotopy colimits

23 votes
1 answer
1k views

Model existence for infinitary logics

22 votes
1 answer
2k views

What are the algebras of the double powerset monad?

20 votes
1 answer
1k views

Non-axiomatisability and ultraproducts

19 votes
5 answers
8k views

Angle brackets for tuples

18 votes
2 answers
3k views

The equivalence of two formulae for the Laplace—Beltrami operator

18 votes
2 answers
4k views

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

17 votes
1 answer
3k views

The projective model structure on chain complexes

16 votes
2 answers
3k views

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

16 votes
1 answer
1k views

What are the universally effective epimorphisms of topological spaces?

16 votes
2 answers
1k views

Coherence for symmetric monoidal categories

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