Let $K/\mathbb{Q}_p$ be a local field, $A$ the ring of integers of K, $\pi$ a uniformizer element for $A$, $F$ an n-dimensional formal group with coefficients in $A$ and $f$ an endomorphism of $F$. When we look at the reduction of $f$ $\pmod \pi$ it has the form $t(x^{p^h})$, where $X^{p^h}=(X_1^{p^h},\dots, X_N^{p^h})$ and the matrix, C(t), of linear coefficients of $t$ is not the zero matrix.

Do you know a proof-or a reference where I can look at- of the following theorem?

Theorem1:$f$ is an isogeny(i.e surjective and finite kernel) iff $c(t)$ is invertible. And the kernel is a porwer of $p$.

Thanks!!!