  Every Salem numbers of degree 4,6,8,12,14 or 16 is the dynamical degree of an
automorphism of a non-projective K3 surface. We define a notion of signature of
an automorphism, and use it to give a necessary and sufficient condition for
Salem numbers of degree 10 and 18 to be realized as the dynamical degree of
such an automorphism. The first part of the paper contains results on
isometries of lattices.
