An Introduction to Homological Algebra
《An Introduction to Homological Algebra》 · 第 345 页 · Definition
For an additive covariant functor \(T:\mathcal A\to\mathcal C\), with \(\mathcal A\) having enough projectives, the functors \(L_nT\) obtained from projective resolutions are called the left derived functors of \(T\).