Let X be a smooth projective curve of genus g≥2 defined over an
algebraically closed field k of characteristic p>0 and let F:X→X1​ be the relative k-linear Frobenius map. We prove (Theorem 1.1) E is a
stable bundle on X1​ with I(E)=(p−1)(2g−2) if and only if E is the
direct image of some stable bundle W on X.Comment: 4 page