Puzzling through exact sequences
A former professor of mine, Ravi Vakil, put together a delightful picturebook explanation for how he likes to schematically visualize exact sequences. It's too delightful not to share, and he kindly let me throw it up here as a guest post (pdf version here).
The target audience probably consists mostly of graduate students in mathematics, or else anyone who already has some familiarity with and reason to care about exact sequences, the snake lemma, cohomology, etc. This piece is not meant to introduce any of those, but in conjunction with traditional texts on the matters, it stands to be a very helpful companion.
Such topics rarely get bedtime stories, or strong visuals, which is part of what makes this such a gem. Enjoy!