Submodule structures of $\mathbb C[s,t]$ over $W(0,b)$ and a new class of irreducible modules over the Virasoro algebra

Abstract

For any $a,b\in\mathbb C$, $W(a,b)$ is the Lie algebra with basis $\{L_m,M_m\,|\,m\in\mathbb Z\}$ and relations $[L_m,L_n]=(n-m)L_{m+n},$ $[L_m,W_n]=(a+n+bm)W_{m+n}$, $[W_m,W_n]=0$ for $m,n\in\mathbb Z$. For any $\lambda\in\mathbb C^*,$ $\alpha\in\mathbb C$, $h:=h(t)\in\mathbb C[t]$, there exists a non-weight module over $W(0,b)$ (resp., $W(0,1)$), denoted by $\Phi(\lambda,\alpha,h)$ (resp. $\Theta(\lambda,h)$), which is defined on the space $\mathbb C[s,t]$ of polynomials on variables $s,t$ and is free of rank one over the enveloping algebra $U(\mathbb C L_0\oplus\mathbb C W_0)$ of $\mathbb C L_0\oplus\mathbb C W_0$. In the present paper, by introducing two sequences of useful operators on $\mathbb C[s,t]$, we determine all submodules of $\mathbb C[s,t]$. We also study submodules of $\mathbb C[s,t]$ regarded as modules over the Virasoro algebra $\mathscr V\!$ (with the trivial action of the center), and prove that these submodules are finitely generated if and only if ${\rm deg}\,h(t)\geq1$. In addition, it is proven that $\Phi(\lambda, \alpha,h)$ is an irreducible $\mathscr V\!$-module if and only if $b=-1$, ${\rm deg}\, h(t)=1$, $\alpha\neq0$. Finally, we obtain a large family of new irreducible modules over the Virasoro algebra $\mathscr V\!$, by taking various tensor products of a finite number of irreducible modules $\Phi(\lambda_i,\alpha_i, h_i)$ for $\lambda_i,\alpha_i\in\mathbb C^*,$ $h_i\in\mathbb C[t]$ with an irreducible $\mathscr V\!$-module $V$, where $V$ satisfies that there exists a nonnegative integer $R_V$ such that $L_m$ acts locally finitely on $V$ for $m\geq R_V$.