Gorenstein–Walter theorem
In mathematics, the Gorenstein–Walter theorem, proved by, states that if a finite group G has a dihedral Sylow 2-subgroup, and O is the maximal normal subgroup of odd order, then G/O is isomorphic to a 2-group, or the alternating group A7, or a subgroup of PΓL2 containing PSL2 for q an odd prime power. Note that A5 ≈ PSL2 ≈ PSL2 and A6 ≈ PSL2.
