The symmetry of finite group schemes, Watanabe type theorem, and the $a$-invariant of the ring of invariants

Abstract

Let $k$ be a field, and $G$ be a $k$-group scheme of finite type. Let $G_{\mathrm{ad}}$ be the $k$-scheme $G$ with the adjoint action of $G$. We call $\lambda_{G,G}=H^0(\mathop{\mathrm{Spec}} k,e^*(\omega_{G_{\mathrm{ad}}}))$ the Knop character of $G$, where $e:\mathop{\mathrm{Spec}} k\rightarrow G_{\mathrm{ad}}$ is the unit element, and $\omega_{G_{\mathrm{ad}}}$ is the $G$-canonical module. We prove that $\lambda_{G,G}$ is trivial in the following cases: (1) $G$ is finite, and $k[G]^*$ is a symmetric algebra; (2) $G$ is finite and \'etale; (3) $G$ is finite and constant; (4) $G$ is smooth and connected reductive; (5) $G$ is abelian; (6) $G$ is finite, and the identity component $G^\circ$ of $G$ is linearly reductive; (7) $G$ is finite and linearly reductive. Let $V$ be a small $G$-module of dimension $n<\infty$. We assume that $\lambda_{G,G}$ is trivial. Let $H=\Bbb G_m$ be the one-dimensional torus, and let $V$ be of degree one as an $H$-module so that $S=\mathop{\mathrm{Sym}} V^*$ is a $\tilde G$-algebra generated by degree one elements, where $\tilde G=G\times H$. We set $A=S^G$. Then we have (i) $\omega_A\cong\omega_S^G$ as $(H,A)$-modules; (ii) $a(A)\leq -n$ in general, where $a(A)$ denotes the $a$-invariant. Moreover, the following are equivalent: (1) The action $G\rightarrow\mathrm{GL}(V)$ factors through $\mathrm{SL}(V)$; (2) $\omega_S\cong S(-n)$ as $(\tilde G,S)$-modules; (3) $\omega_S\cong S$ as $(G,S)$-modules; (4) $\omega_A\cong A(-n)$ as $(H,A)$-modules; (5) $A$ is quasi-Gorenstein; (6) $A$ is quasi-Gorenstein and $a(A)=-n$; (7) $a(A)=-n$. This partly generalizes recent results of Liedtke--Yasuda arXiv:2304.14711v2 and Goel--Jeffries--Singh arXiv:2306.14279v1.