Homology, Homotopy and Applications

Volume 5 (2003)

Number 1

Group extensions and automorphism group rings

Pages: 53 – 70

DOI: http://dx.doi.org/10.4310/HHA.2003.v5.n1.a3


John Martino (Department of Mathematics, Western Michigan University, Kalamazoo, Mich., U.S.A.)

Stewart Priddy (Department of Mathematics, Northwestern University, Evanston, Illinois, U.S.A.)


We use extensions to study the semi-simple quotient of the group ring $\mathbf{F}_pAut(P)$ of a finite $p$-group $P$. For an extension $E: N \to P \to Q$, our results involve relations between $Aut(N)$, $Aut(P)$, $Aut(Q)$ and the extension class $[E]\in H^2(Q, ZN)$. One novel feature is the use of the intersection orbit group $\Omega([E])$, defined as the intersection of the orbits $Aut(N)\cdot[E]$ and $Aut(Q)\cdot [E]$ in $H^2(Q,ZN)$. This group is useful in computing $|Aut(P)|$. In case $N$, $Q$ are elementary Abelian $2$-groups our results involve the theory of quadratic forms and the Arf invariant.


automorphism group, extension class, semi-simple quotient, stable splittings

2010 Mathematics Subject Classification

Primary 20J06. Secondary 55P42.

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