Even-involution coset fixed-point lemma
= Even-involution coset fixed-point lemma
Suppose a finite group $G$ has no index-two subgroup, $P$ is a Sylow $2$-subgroup, $H$ has index two in $P$, and $x\in G$ has order two. The sign of the action on $G/H$ defines a homomorphism $G\to\{\pm1\}$ and is therefore trivial. Since $[G:H]\equiv2\pmod4$, a fixed-point-free involution would be an odd number of transpositions. Thus $x$ fixes a coset $gH$, equivalently $g^{-1}xg\in H$.