Let
X be a (projective, geometrically irreducible, nonsingular) algebraic curve of genus
defined over an algebraically closed field
K of odd characteristic
p. Let
Aut(X) be the group of all automorphisms of
X which fix
K element-wise. It is known that if
then the
p-rank (equivalently, the Hasse-Witt invariant) of
X is zero. This raises the problem of
... [Show full abstract] determining the (minimum-value) function f(g) such that whenever then X has zero p-rank. For {\em{even}} g we prove that . The {\em{odd}} genus case appears to be much more difficult although, for any genus , if Aut(X) has a solvable subgroup G such that then X has zero p-rank and G fixes a point of X. Our proofs use the Hurwitz genus formula and the Deuring Shafarevich formula together with a few deep results from finite group theory characterizing finite simple groups whose Sylow 2-subgroups have a cyclic subgroup of index 2. We also point out some connections with the Abhyankar conjecture and the Katz-Gabber covers.