Enumeration of diagonally colored Young diagrams

In this note we give a new proof of a closed formula for the multivariable generating series of diagonally colored Young diagrams. This series also describes the Euler characteristics of certain Nakajima quiver varieties. Our proof is a direct combinatorial argument, based on Andrews' work on generalized Frobenius partitions. We also obtain representations of these series in some particular cases as infinite products.Comment: Final version, 12 pages. To appear in Monatshefte f\"ur Mathemati

Recollement from DG quotients

We review a recollement for derived categories of DG categories arising from Drinfeld quotients. As an application we show that an exact sequence well-known in K-theory descends to numerical K-groups provided that either the quotient or the category we take the quotient with has a Serre functor, and if either the quotient functor preserves compactness or if the K-group of the quotient is torsion-free.Comment: 7 pages, comments are welcome. arXiv admin note: text overlap with arXiv:2105.1333

Slicing up multigraded linear series

Multigraded linear series generalize the classical morphism to the linear series of a basepoint-free line bundle on a scheme. We investigate the collection of the natural cornering morphisms into elementary bigraded linear series obtained from direct summands of the original globally generated vector bundle. Our main result is a condition on the injectivity of the product morphism. We apply our result in three examples: modules over the reconstruction algebra, equivariant Hilbert and Quot schemes of quotient stacks and Kapranov's tilting bundle over the Grassmannian.Comment: 11 pages. Comments are welcom

Euler characteristics of Hilbert schemes of points on simple surface singularities

We study the geometry and topology of Hilbert schemes of points on the orbifold surface [C^2/G], respectively the singular quotient surface C^2/G, where G is a finite subgroup of SL(2,C) of type A or D. We give a decomposition of the (equivariant) Hilbert scheme of the orbifold into affine space strata indexed by a certain combinatorial set, the set of Young walls. The generating series of Euler characteristics of Hilbert schemes of points of the singular surface of type A or D is computed in terms of an explicit formula involving a specialized character of the basic representation of the corresponding affine Lie algebra; we conjecture that the same result holds also in type E. Our results are consistent with known results in type A, and are new for type D.Comment: 57 pages, final version. To appear in European Journal of Mathematic

GG-fixed Hilbert schemes on K3K3 surfaces, modular forms, and eta products

Let $X$ be a complex $K3$ surface with an effective action of a group $G$ which preserves the holomorphic symplectic form. Let $$Z_{X,G}(q) = \sum_{n=0}^{\infty} e\left(\operatorname{Hilb}^{n}(X)^{G} \right)\, q^{n-1}$$ be the generating function for the Euler characteristics of the Hilbert schemes of $G$-invariant length $n$ subschemes. We show that its reciprocal, $Z_{X,G}(q)^{-1}$ is the Fourier expansion of a modular cusp form of weight $\frac{1}{2} e(X/G)$ for the congruence subgroup $\Gamma_{0}(|G|)$. We give an explicit formula for $Z_{X,G}$ in terms of the Dedekind eta function for all 82 possible $(X,G)$. The key intermediate result we prove is of independent interest: it establishes an eta product identity for a certain shifted theta function of the root lattice of a simply laced root system. We extend our results to various refinements of the Euler characteristic, namely the Elliptic genus, the Chi-$y$ genus, and the motivic class.Comment: Minor edits and reference update

Rigid ideal shaves and modular forms

Let X be a complex smooth quasi-projective surface acted upon by a finite group G such that the quotient X/G has singularities only of ADE type. We obtain an explicit expression for the generating series of the Euler characteristics of the zero-dimensional components in the moduli space of zero-dimensional subschemes of X invariant under the action of G. We show that this generating series (up to a suitable rational power of the formal variable) is a holomorphic modular form