摘要
Let X be a smooth projective curve of genus g≥2 defined over an algebraically closed field k of characteristic p>0. Let MX(r) be the moduli space of semi-stable rankr vector bundles with fixed trivial determinant. The relative Frobenius map F:X→X1 induces by pull-back a rational map V:MX1(r)→MX(r). We determine the equations of V in the following two cases (1) (g,r,p)=(2,2,2) and X nonordinary with Hasse–Witt invariant equal to 1 (see math.AG/0005044 for the case X ordinary), and (2) (g,r,p)=(2,2,3). We also show the existence of base points of V, i.e., semi-stable bundles E such that F*E is not semi-stable, for any triple (g,r,p).