Multigraded Hilbert series of invariants, covariants, and symplectic quotients for some rank $1$ Lie groups

TL;DR

This paper calculates multigraded Hilbert series for invariants and covariants of circle and orthogonal group representations, including symplectic quotients, providing algorithms and examples for these computations.

Contribution

It introduces methods to compute multigraded Hilbert series for invariants and covariants of rank 1 Lie groups, extending to symplectic quotients and semidirect products.

Findings

Computed univariate and multigraded Hilbert series for specific group actions

Provided algorithms for Hilbert series computation

Illustrated techniques with examples and Laurent coefficient calculations

Abstract

We compute univariate and multigraded Hilbert series of invariants and covariants of representations of the circle and orthogonal group $\operatorname{O}_2$. The multigradings considered include the maximal grading associated to the decomposition of the representation into irreducibles as well as the bigrading associated to a cotangent-lifted representation, or equivalently, the bigrading associated to the holomorphic and antiholomorphic parts of the real invariants and covariants. This bigrading induces a bigrading on the algebra of on-shell invariants of the symplectic quotient, and the corresponding Hilbert series are computed as well. We also compute the first few Laurent coefficients of the univariate Hilbert series, give sample calculations of the multigraded Laurent coefficients, and give an example to illustrate the extension of these techniques to the semidirect product of the circle by other finite groups. We describe an algorithm to compute each of the associated Hilbert series.