User Lennart Meier

15 votes

Can anyone give me a good example of two interestingly different ordinary cohomology theories?

answered May 29, 2010 at 20:16

14 votes

Modern source for spectra (including ring spectra)

answered Nov 23, 2011 at 20:07

14 votes

Accepted

Does $\mathfrak{N}_4$ contain at least four distinct elements?

answered Dec 18, 2015 at 16:24

13 votes

Small ideas that became big

answered Sep 22, 2020 at 11:20

13 votes

Theorems that are 'obvious' but hard to prove

answered Jan 19, 2011 at 21:18

13 votes

Accepted

generalized universal coefficient sequence

answered Sep 7, 2015 at 7:44

13 votes

Weighted projective spaces as stacks

answered Feb 1, 2016 at 18:17

12 votes

Accepted

Growth of stable homotopy groups of spheres

answered Jan 30, 2018 at 12:19

12 votes

Problems where we can't make a canonical choice, solved by looking at all choices at once

answered Oct 10, 2013 at 2:28

12 votes

Historical transition from classical homotopy to modern homotopy theory

answered May 3, 2010 at 14:33

12 votes

Accepted

Rational homotopy invariance of algebraic $K$-theory

answered Jun 28, 2020 at 15:47

12 votes

Conceptual proof of classification of surfaces?

answered Feb 20, 2020 at 17:57

11 votes

Accepted

Adams e-invariant

answered Aug 29, 2014 at 2:18

10 votes

Accepted

What's an example of 2 elliptic curves with the same ground ring s.t. their associated cohomology theories detect different things?

answered Mar 30, 2015 at 18:07

10 votes

What are some examples of narrowly missed discoveries in the history of mathematics?

answered Apr 10, 2016 at 13:52

10 votes

Extremely messy proofs

answered Apr 19, 2015 at 3:06

10 votes

Proof that the homotopy category of a stable $\infty$-category is triangulated

answered Feb 13, 2011 at 13:18

9 votes

Accepted

What is the topology on hom-sets of spectra?

answered Aug 10, 2011 at 8:54

9 votes

Mayer-Vietoris implies Excision

answered May 22, 2012 at 9:08

9 votes

Accepted

Homology is computable because it is stable under suspension

answered Oct 7, 2010 at 8:37

9 votes

(Homotopy theory) When does the 2 of 3 property not imply 2 of 6?

answered Sep 27, 2016 at 8:52

9 votes

What recent programmes to alter highly-entrenched mathematical terminology have succeeded, and under what conditions do they tend to succeed or fail?

answered Oct 28, 2014 at 22:38

9 votes

Most intriguing mathematical epigraphs

answered Oct 5, 2015 at 7:39

9 votes

Second homotopy group of the mod 2 Moore spectrum

answered Aug 22, 2013 at 20:08

8 votes

relation between sheaf of hom and hom of sheaf

answered Sep 26, 2013 at 15:49

8 votes

Homotopy groups other than $\pi_1$ : what are they good for?

answered Nov 5, 2013 at 21:20

8 votes

Is there a good way to understand the free loop space of a sphere?

answered Nov 23, 2013 at 4:03

8 votes

Colimits of cofibrations and homotopy colimits

answered Jan 25, 2017 at 20:49

8 votes

Accepted

Good introductory references on moduli (stacks), for arithmetic objects

answered Aug 28, 2018 at 5:46

8 votes

Algebraic topology beyond the basics: any texts bridging the gap?

answered Nov 12, 2012 at 11:45

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94 Answers

Can anyone give me a good example of two interestingly different ordinary cohomology theories?

Modern source for spectra (including ring spectra)

Does $\mathfrak{N}_4$ contain at least four distinct elements?

Small ideas that became big

Theorems that are 'obvious' but hard to prove

generalized universal coefficient sequence

Weighted projective spaces as stacks

Growth of stable homotopy groups of spheres

Problems where we can't make a canonical choice, solved by looking at all choices at once

Historical transition from classical homotopy to modern homotopy theory

Rational homotopy invariance of algebraic $K$-theory

Conceptual proof of classification of surfaces?

Adams e-invariant

What's an example of 2 elliptic curves with the same ground ring s.t. their associated cohomology theories detect different things?

What are some examples of narrowly missed discoveries in the history of mathematics?

Extremely messy proofs

Proof that the homotopy category of a stable $\infty$-category is triangulated

What is the topology on hom-sets of spectra?

Mayer-Vietoris implies Excision

Homology is computable because it is stable under suspension

(Homotopy theory) When does the 2 of 3 property not imply 2 of 6?

What recent programmes to alter highly-entrenched mathematical terminology have succeeded, and under what conditions do they tend to succeed or fail?

Most intriguing mathematical epigraphs

Second homotopy group of the mod 2 Moore spectrum

relation between sheaf of hom and hom of sheaf

Homotopy groups other than $\pi_1$ : what are they good for?

Is there a good way to understand the free loop space of a sphere?

Colimits of cofibrations and homotopy colimits

Good introductory references on moduli (stacks), for arithmetic objects

Algebraic topology beyond the basics: any texts bridging the gap?