J. Algebra 16 1970 436--441.

Erratum

p.400 In the second line of the proof of Theorem 2, replace $\mathrm{Ext}_{\mathcal{G}}^{1}$ with $\mathrm{Ext}_{\mathcal{G}}^{2}$. Also, in the commutative diagram, the arrow $s$ should point in the opposite direction.