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Invariants of pure 2-dimensional sheaves inside threefolds and modular forms

Motivated by S-duality modularity conjectures in string theory, we study the Donaldson-Thomas type invariants of pure 2-dimensional sheaves inside a nonsingular threefold X in three different situations: (1). X is a K3 fibration over a curve. We study the Donaldson-Thomas invariants of the 2 dimensional Gieseker stable sheaves in X supported on the fibers. Analogous to the Gromov-Witten theory formula established in the work of Maulik-Pandharipande, we express these invariants in terms of the Euler characteristic of the Hilbert scheme of points on the K3 surface and the Noether-Lefschetz numbers of the fibration, and prove that the invariants have modular properties. (2). X is the total space of the canonical bundle of P^2. We study the generalized Donaldson-Thomas invariants defined by Joyce-Song of the moduli spaces of the 2-dimensional Gieseker semistable sheaves on X with first Chern class equal to k times the class of the zero section of X. When k=1,2 or 3, and semistability implies stability, we express the invariants in terms of known modular forms. We prove a combinatorial formula for the invariants when k=2 in the presence of the strictly semistable sheaves, and verify the BPS integrality conjecture of Joyce-Song in some cases. (3). (Joint with Richard Thomas) X is a Calabi-Yau threefold and L is a sufficiently positive line bundle. We define new invariants counting a restricted class of 2-dimensional torsion sheaves, enumerating pairs (Z,H) in X where H is a member of the linear system |L| and Z is a 1-dimensional subscheme of H. The associated sheaf is the ideal sheaf of Z in H, pushed forward to X and considered as a certain Joyce-Song pair in the derived category of X. We express these invariants in terms of the MNOP invariants of X.