Published online by Cambridge University Press: 20 November 2018
We prove the congruence relation for the $\bmod -p$ reduction of Shimura varieties associated with a unitary similitude group $GU(n\,-\,1,\,1)$ over $\mathbb{Q}$ when $p$ is inert and $n$ odd. The case when $n$ is even was obtained by T. Wedhorn and O. Bültel, as a special case of a result of B. Moonen, when the $\mu$–ordinary locus of the $p$–isogeny space is dense. This condition fails in our case. We show that every supersingular irreducible component of the special fiber of $p-I\text{sog}$ is annihilated by a degree one polynomial in the Frobenius element $F$, which implies the congruence relation.