Co-Higgs bundles on P1

Abstract. Co-Higgs bundles are Higgs bundles in the sense of Simp-son, but with Higgs fields that take values in the tangent bundle instead of the cotangent bundle. Given a vector bundle on P1, we find necessary and sufficient conditions on its Grothendieck splitting for it to admit a stable Higgs field. We characterize the rank-2, odd-degree moduli space as a universal elliptic curve with a globally-defined equation. For ranks r = 2, 3, 4, we explicitly verify the conjectural Betti numbers emerging from the recent work of Chuang, Diaconescu, Pan, and Mozgovoy on the ADHM formula. We state the result for r = 5. Content