Membranes and sheaves

Our goal in this paper is to discuss a conjectural correspondence between the enumerative geometry of curves in Calabi-Yau 5-folds Z and 1-dimensional sheaves on 3-folds X that are embedded in Z as fixed points of certain C-x -actions. In both cases, the enumerative information is taken in equivariant K-theory, where the equivariance is with respect to all automorphisms of the problem. In Donaldson-Thomas theory, one sums over all Euler characteristics with a weight (-q)(chi), where q is a parameter, informally referred to as the boxcounting parameter. The main feature of the correspondence is that the 3-dimensional boxcounting parameter q becomes in dimension 5 the equivariant parameter for the C-x -action that de fi nes X inside Z. The 5-dimensional theory effectively sums up the q-expansion in the Donaldson-Thomas theory. In particular, it gives a natural explanation of the rationality (in q) of the DT partition functions. Other expected as well as unexpected symmetries of the DT counts follow naturally from the 5-dimensional perspective. These involve choosing different C-x -actions on the same Z, and thus relating the same 5-dimensional theory to different DT problems. The important special case Z = X x C-2 is considered in detail in Sections 7 and 8. If X is a toric Calabi-Yau 3-fold, we compute the theory in terms of a certain index vertex. We show that the re fined vertex found combinatorially by Iqbal, Kozcaz, and Vafa is a special case of the index vertex.