Whittaker modules for $\widehat{\mathfrak gl}$ and $\mathcal W_{1+ \infty}$-modules which are not tensor products

Abstract: We consider the Whittaker modules $M_{1}(\lambda,\mu)$ for the Weyl vertex algebra $M$, constructed in arXiv:1811.04649, where it was proved that these modules are irreducible for each finite cyclic orbifold $M^{\Bbb Z_n}$. In this paper, we consider the modules $M_{1}(\lambda,\mu)$ as modules for the ${\Bbb Z}$-orbifold of $M$, denoted by $M^0$. $M^0$ is isomorphic to the vertex algebra $\mathcal W_{1+\infty, c=-1} = \mathcal M(2) \otimes M_1(1)$ which is the tensor product of the Heisenberg vertex algebra $M_1(1)$ and the singlet algebra $\mathcal M(2)$. Furthermore, these modules are also modules of the Lie algebra $\widehat{\mathfrak gl}$ with central charge $c=-1$. We prove they are reducible as $\widehat{\mathfrak gl}$-modules (and therefore also as $M^0$-modules), and we completely describe their irreducible quotients $L(d,\lambda,\mu)$. We show that $L(d,\lambda,\mu)$ in most cases are not tensor product modules for the vertex algebra $ \mathcal M(2) \otimes M_1(1)$. Moreover, we show that all constructed modules are typical in the sense that they are irreducible for the Heisenberg-Virasoro vertex subalgebra of $\mathcal W_{1+\infty, c=-1}$.