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96 Chapter 7. Special Schreier and special homogeneous surjections
For the same reasons as in Mon, this operation gives SExt(Y,A) an abelian group structure. The zero of this group is just the isomorphic class of the split extension corresponding to the direction (A Y, Y, pY , ιY ). The inverse of a
special Schreier (resp. homogeneous) extension in SRng: A // k // X f // // Y isgivenby: A // −k //X f //// Y .
