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Homotopy theory, homological algebra, algebraic treatments of manifolds.

15 votes
1 answer
6k views

Exactness of 2nd-Order Differential Equations via Differential Forms

This (probably very elementary) question came up the last time I taught differential equations, and I've been toying with it for a while with no success: A 1st-order differential equation $M(x,y)dx+N …
Cam McLeman's user avatar
  • 8,467
9 votes

Essential theorems in group (co)homology

Shapiro's lemma and dimension-shifting. Cohomology of cyclic groups and Herbrand quotients. It would be helpful to know what you need to know group cohomology for. If you have an interest in pr …
31 votes
3 answers
3k views

Why aren't there more classifying spaces in number theory?

Much of modern algebraic number theory can be phrased in the framework of group cohomology. (Okay, this is a bit of a stretch -- much of the part of algebraic number theory that I'm interested in...) …
Cam McLeman's user avatar
  • 8,467
9 votes

Geometric interpretation of the lower central series for the fundamental group?

A special case you might find informative: If $L$ is a link in $S^3$, then Chen-Milnor theory gives you a presentation of the link group $\pi=\pi_1(S^3-L)$ modulo some deeper terms of the the lower c …
Cam McLeman's user avatar
  • 8,467