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James
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5 votes
1 answer
471 views

Why is a pseudo-abelian category idempotent complete?

4 votes
1 answer
564 views

Equivalent definitions of abelian categories - reference request

3 votes
2 answers
209 views

Yoneda Lemma - why is $[\mathscr A^{\text{op}}, \textbf{Set}](H_A,X)$ a set?

2 votes
1 answer
43 views

Prove objects are direct sums in semisimple categories

2 votes
1 answer
232 views

Integration obeys countable subadditivity?

2 votes
2 answers
4k views

Why is homology invariant under deformation retraction/homotopy equivalence?

2 votes
1 answer
694 views

The degree of a map is the induced multiplication on any cohomology theory

1 vote
1 answer
145 views

Degree of a constant map $f : S^2 \to S^2$

1 vote
1 answer
211 views

What does it mean for this exact sequence to split?

1 vote
1 answer
52 views

K theory and maps into unitary group - reference request

1 vote
0 answers
42 views

External product is not injective in real K theory

0 votes
0 answers
42 views

Is $yf-1$ prime in $R[y]$ if $f$ not nilpotent?