Maximum likelihood degree of surjective rational maps

With any \emph{surjective rational map} $f: \mathbb{P}^n \dashrightarrow \mathbb{P}^n$ of the projective space we associate a numerical invariant (\emph{ML degree}) and compute it in terms of a naturally defined vector bundle $E_f \longrightarrow \mathbb{P}^n$.Comment: 4 pages; the text has been revised following the referee's suggestion