Schur multiplier
（重定向自Stem extension）
![A projective representation of G can be pulled back to a linear representation of a central extension C of G.](/Images/godic/202502/13/Projective-representation-lifting.svg1531.png")
In mathematical group theory, the Schur multiplier or Schur multiplicator is the second homology group H2(G, Z) of a group G. It was introduced by IssaiSchur (1904) in his work on projective representations.