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(Co)chain complexes, abelian Categories, (pre)sheaves, (co)homology in various (possibly highly generalized) settings, spectra, derived functors, resolutions, spectral sequences, homotopy categories. Chain complexes in an abelian category form the heart of homological algebra.

7 votes

From chain complex to simplicial abelian group

This is the Dold-Kan correspondence.
Robin Chapman's user avatar
5 votes
Accepted

Notation for algebras

It's pretty standard to use $\bigwedge(V)$ or $\Lambda(V)$ for the exterior algebra on a vector space $V$ and $\bigwedge^k(V)$ or $\Lambda^k(V)$ for the $k$-th graded part. For symmetric algebras $S(V …
Robin Chapman's user avatar
8 votes
Accepted

Projective dimension

No. A submodule of a free module need not have finite projective dimension. As a simple example let $R=\mathbb{Z}/p^2\mathbb{Z}$. The free module $R$ has a submodule $p\mathbb{Z}/p^2\mathbb{Z}\cong\ma …
Robin Chapman's user avatar