Lemma 15.84.12. Let R be a ring. Let K \in D(R) be given by a two term complex of the form R^{\oplus n} \to R^{\oplus n}. Denote A \in \text{Mat}(n \times n, R) the matrix of the differential. Then \det (a) : K \to K is zero in D(R).
Proof. Omitted. Good exercise. \square
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