Schur multiplier (reviewed)

If $G$ is a finite group then the Schur multiplier of $G$ is the homology group $H_2(G, \Z)$. It is the maximal kernel of a stem extension of $G$. In the data, the Schur multiplier is represented as a list of invariant factors; e.g., [2,4,12] corresponds to the abelian group $C_2 \times C_4 \times C_{12}$.