Planar rank-one sheaves on P3, obstruction bundles, and divisor-supported Donaldson–Thomas series

Abstract

Let $X=\mathbb{P}^3$ and let \[ \alpha_n=(0,1,-\tfrac12,\tfrac16-n)\in H^{\mathrm{even}}(X,\mathbb{Q}) \] with respect to the basis $1,H,H^2,H^3$, where $H=c_1(\mathcal{O}_X(1))$. We prove that every Gieseker semistable sheaf on $X$ with Chern character $\alpha_n$ is stable and uniquely of the form $\iota_{P*}\mathcal{I}_Z$ for a plane $P\subset X$ and a length-$n$ subscheme $Z\subset P$. Hence the coarse stable-sheaf moduli space is the relative Hilbert scheme of points on the universal plane over the dual projective space $X^{\vee}$. Using the standard perfect obstruction theory for stable sheaves on a Fano threefold, we identify the obstruction bundle with a relative Carlsson--Okounkov twisted tangent bundle and obtain a closed product formula \[ \sum_{n\ge 0} \Gamma_n q^n=\prod_{m\ge 1}(1-q^m)^{-7} \] for the natural point-inserted two-dimensional Donaldson--Thomas invariants on $\mathbb{P}^3$. We then develop the analogous divisor-supported theory for a smooth divisor $D$ in a smooth projective Fano threefold, distinguishing moving and rigid divisors. For a rigid divisor satisfying $H^1(D,\mathcal{O}_D)=0$, the divisor-supported moduli component is a Hilbert scheme of points on $D$, its obstruction bundle is the twisted tangent bundle $\mathsf{T}_D^{[n]}(N_{D/X})$, and its generating series is \[ \prod_{m\ge 1}(1-q^m)^{-(c_2(X)\cdot D+D^3)}. \] We work out the examples of the exceptional divisor in $\operatorname{Bl}_p\mathbb{P}^3$ and of a rigid section in a Fano $\mathbb{P}^1$-bundle over $\mathbb{P}^1\times\mathbb{P}^1$.