Projection problem interpretation Problem: Describe the projective transformations of $FP^n$ that preserve the $FP^{n−1}$ at infinity given by $x_0 = 0$. 
My attempt: We have to preserve the points with first coordinate $0$, i.e. the transformation maps $[0, x_1, x_2...x_n]$ to $[0, y_1, ...y_n]$. I can see why we can represent $FP^n$ as the disjoint union of $F^n$ and $FP^{n-1}$ and the latter is basically what we want to preserve. Now I also know that a projective transformation is given by $v$   -> $[Tv]$, where $T:R^n$ -> $R^n$ is injective. I just can't seem to finalize the argument. 
Thank you in advance
Rick M

Hint
Let's work with vector spaces : if $V = F^{n+1}$ with basis $B = (e_0, e_1, \dots, e_n)$ you are asking for matrices $g \in GL(V)$ such that $g(W) = W$ where $W$ has basis $(e_1, \dots, e_n)$. Do you agree ? Do you see how to finish ?
