A new proof of the mathfrak{sl}₂ action on the triplet vertex algebra

Let $\mathcal {W}(p)$ be the triplet vertex algebra of central charge $c_{p}=1-\frac{6(p-1)^{2}}{p}$, $p\geq2$. As a Virasoro module, we have $$\mathcal {W}(p)=\bigoplus_{n=0} ^{\infty}(2n+1) L(c_{p}, n^{2}p+np-n).$$ It was pointed out in \cite{am1} that $\mathcal {W}(p)$ admits an action of $\mathfrak{sl}_{2}$. In this paper we give a combinatorics description of $\mathcal {W}(p)$, from which the action of $\mathfrak{sl}_{2}$ follows quite directly. In the end of this paper we give similar descriptions of the invariant subalgebra $\mathcal {W}(p)^{\Gamma}$, these will be useful for the characterizations of the exceptional vertex operator algebras of central charge $1$ in forthcoming papers. We also hope to extend the method of this paper to subalgebra of lattice vertex operator algebras of higher rank.