Published online by Cambridge University Press: 25 June 2014
We classify all (abstract) homomorphisms from the group $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{\sf PGL}_{r+1}(\mathbf{C})$ to the group
${\sf Bir}(M)$ of birational transformations of a complex projective variety
$M$, provided that
$r\geq \dim _\mathbf{C}(M)$. As a byproduct, we show that: (i)
${\sf Bir}(\mathbb{P}^n_\mathbf{C})$ is isomorphic, as an abstract group, to
${\sf Bir}(\mathbb{P}^m_\mathbf{C})$ if and only if
$n=m$; and (ii)
$M$ is rational if and only if
${\sf PGL}_{\dim (M)+1}(\mathbf{C})$ embeds as a subgroup of
${\sf Bir}(M)$.