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Questions about the branch of algebra that deals with groups.
5
votes
Accepted
Injectivity of the cohomology map induced by some projection map
Ok, I will follow Fernando's advice and post an answer. I learned the computation below from the beginning of Pin(2)-equivariant Seiberg--Witten Floer homology and the triangulation conjecture.
The gr …
54
votes
Is SO(4) a subgroup of SU(3)?
No. There is probably a straightforward representation-theoretic argument, but I am too ignorant of the subject to give one, so here is a topological argument.
If $H \subset G$ are Lie groups with $H$ …
9
votes
Classifying space of semidirect product of groups
I am adding my comment as an answer.
Every extension of groups $1 \to H \to G \to K \to 1$ corresponds to a fibration $$BH \to BG \to BK,$$ or a little more precisely at the space level $$EG/H \to (E …
2
votes
Accepted
Monoid of continuous self-maps of (real) surfaces
Here's some sort of geometric description of maps of degree 0. I don't know how realistically you can find a generators-and-relations description of the semigroup of such maps.
It is a theorem due to …