Methods of Homological Algebra
《Methods of Homological Algebra》 · 第 120 页 · Chapter II, Section 5.7
A commutative diagram with exact rows \(0\to X_1\to X_2\to X_3\to0\) and \(0\to Y_1\to Y_2\to Y_3\to0\) induces the standard exact sequence of kernels and cokernels with a connecting morphism.