Weighted Euler characteristic of the moduli space of higher rank Joyce–Song pairs

Abstract

The invariants of rank 2 Joyce–Song semistable pairs over a Calabi–Yau threefold were computed in Sheshmani (Illinois J Math 59(1):55–83, 2016), using the wall-crossing formula of Joyce and Song (A Theory of Generalized Donaldson–Thomas Invariants. Memoirs of American Mathematical Society, American Mathematical Society, Providence, 2012), and Kontsevich and Soibelman (Stability structures, motivic Donaldson–Thomas invariants and cluster transformations, arXiv:0811.2435, 2008). Such wallcrossing computations often depend on the combinatorial properties of certain elements of a Hall-algebra [these are the stack functions defined by Joyce (Adv Math 210(2):635–706, 2007)]. These combinatorial computations become immediately complicated and hard to carry out, when studying higher rank stable pairs with rank > 2. The main purpose of this article is to introduce an independent approach to computation of rank 2 stable pair invariants, without applying the wallcrossing formula and rather by stratifying their corresponding moduli space and directly computing the weighted Euler characteristic of the strata. This approach may similarly be used to avoid complex combinatorial wallcrossing calculations in rank > 2 cases.