Birational geometry of compactifications of Drinfeld half-spaces over a finite field

We study compactifications of Drinfeld half-spaces over a finite field. In particular, we construct a purely inseparable endomorphism of Drinfeld’s half-space $\mathrm{\Omega }(V)$ over a finite field $k$ that does not extend to an endomorphism of the projective space $P(V)$. This should be compared with theorem of R'emy, Thuillier and Werner that every $k$-automorphism of $\Omega (V)$extendstoa$k$-automorphismof$P (V)$. Our construction uses an inseparable analogue of the Cremona transformation. We also study foliations on Drinfeld’s half-spaces. This leads to various examples of interesting varieties in positive characteristic. In particular, we show a new example of a non-liftable projective Calabi-Yau threefold in characteristic $2$ and we show examples of rational surfaces with klt singularities, whose cotangent bundle contains an ample line bundle.