# Defect groups and subgroups

I would like to prove the following statement from Alperin's Local representation theory, p101:

Lemma Let $b$ be a block of the subgroup $H$ of $G$ and let $D$ be a defect group of $b$. If $b^G$ is defined, i.e. if there exists a unique block $b^G$ of $G$ such that $b\mid B_{H\times H}$, then $D$ is contained in a defect group of $b^G$.

Proof. Let $E$ be a defect group of $B=b^G$, so that $B$ has vertex $\delta(E)$ (as a $k(G\times G)$-module). The proof comes down to the following statement which I don't understand: since $b\mid B_{H\times H}$, it follows from Mackey's theorem that the vertex $\delta(D)$ of $b$ is conjugate in $G\times G$ with a subgroup of $\delta(E)$.

Would someone be able to explain this statement to me, i.e. how Mackey's theorem is used here? I understand that we need to prove that $b$ is relatively $\delta(E)$-projective.

Cheers.

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