= Contravariant right derived functor
{title2=$R^iF(M)=H^i(F(P_\bullet))$}
For an additive contravariant <left-exact functor>, apply it to a <projective resolution> of $M$. Reversal of the arrows gives a <cochain complex>. Comparison maps lift using projectivity, and any two lifts are chain homotopic, so its cohomology is independent of the resolution. The functors are contravariant, vanish on projective modules in positive degrees, and send $0\to M'\to M\to M''\to0$ to $0\to F(M'')\to F(M)\to F(M')\to R^1F(M'')\to\cdots$. Degreewise split resolutions from the <horseshoe lemma> yield this sequence.
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