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Let X be a smooth projective curve and A, B and D stable vector bundles on X. Here we study the stratification by the rank of elements of H0 (X, Hom(A, B)) and the properties of the composition map H0 (X, Hom(A, B)) x H0 (X, Hom(B, D)) → H0 (X, Hom(A, D)) sending (ƒ, h) to h ⸰ ƒ.
Let Y be an integral projective curve with g := pa(Y) > 4. What is the minimal integer d such that there is a degree d non-degenerate morphism f Y - Pr? What happens if we look oniy for separable morphisms ? Assume that Y is either not Gorenstein or with oniy planar singularities. Here we give a rough classification of the cases in which d > g + r and several related examples.
Here we study the birational structure of "exceptional" irreducible components of moduli schemes of rank 2 vector bundles on algebraic surfaces with negative Kodaira dimension. In particular for every rational surface X prove the existence of several such components which are rational.
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