Proper Kepka classes (Q1065910)

Let P be a non-empty class of modules closed under extensions. Let K denote the class of short exact sequences \(E=0\to A\to B\to C\to 0\) such that \(E=f^*(F)\) for some short exact sequence \(F=0\to A\to Q\to M\to 0\), where \(M\in P\). In the paper, there is shown that K is a proper class of short exact sequences, i.e. a purity in other terminology (this result was known in a weaker form, namely for P closed under submodules).