FRAMED SHEAVES ON PROJECTIVE STACKS UGO BRUZZO§ ‡ AND FRANCESCO SALA¶? with an appendix by MATTIA PEDRINI§‡

ABSTRACT. Given a normal projective irreducible stack X over an algebraically closed field of character-istic zero we consider framed sheaves on X, i.e., pairs (E, φE), where E is a coherent sheaf on X and φE is a morphism from E to a fixed coherent sheaf F. After introducing a suitable notion of (semi)stability, we construct a projective scheme, which is a moduli space for semistable framed sheaves with fixed Hilbert poly-nomial, and an open subset of it, which is a fine moduli space for stable framed sheaves. If X is a projective irreducible orbifold of dimension two and F a locally free sheaf on a smooth divisor D ⊂ X satisfying cer-tain conditions, we consider (D,F)-framed sheaves, i.e., framed sheaves (E, φE) with E a torsion-free sheaf which is locally free in a neighborhood of D, and φE |D an isomorphism. These pairs are µ-stable for a suitable choice of a parameter entering the (semi)stability condition, and of the polarization of X. This implies the existence of a fine moduli space parameterizing isomorphism classes of (D,F)-framed sheaves on X with fixed Hilbert polynomial, which is a quasi-projective scheme. In an appendix we develop the example of stacky Hirzebruch surfaces. This is the first paper of a project aimed to provide an algebro-geometric approach to the study of gauge theories on a wide class of 4-dimensional Riemannian manifolds by means of framed sheaves on “stacky ” compactifications of them. In particular, in a subsequent paper [20] these results are used to stud