Let $0 \to \mathbb Z/2\mathbb Z \to \mathbb Z/4\mathbb Z\to \mathbb Z/2\mathbb Z \to 0$ be a short exact sequence of complexes concentrated in degree $0$. How can I prove that it cannot be made into an exact trianle?
2026-04-03 01:08:39.1775178519
An example of short exact sequence which is not exact triangle.
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