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.