Let $(E , \omega)$ be a symplectic vector space of dimension $n = 2m$. Let $P = \operatorname { Vect} ( e_1, f_1 )$ with $\omega(e_1,f_1)=1$, and $v = \delta \circ u$ where $u \in \operatorname{Symp}_{\omega}(E)$ and $\delta$ is a composition of symplectic transvections with $v|_P = \mathrm{id}_P$. Show that $P ^ { \omega }$ is stable under $v$.