Ito's conjecture and the coset construction for {mathcal W}^k(mathfrak{sl}(3|2))

Many ${\mathcal W}$-(super)algebras which are defined by the generalized Drinfeld-Sokolov reduction are also known or expected to have coset realizations. For example, it was conjectured by Ito that the principal ${\mathcal W}$-superalgebra ${\mathcal W}^k(\mathfrak{sl}(n+1|n))$ is isomorphic to the coset of $V^{l+1}(\mathfrak{gl}_n)$ inside $V^{l}(\mathfrak{sl}_{n+1}) \otimes {\mathcal E}(n)$ for generic values of $l$. Here ${\mathcal E}(n)$ denotes the rank $n$ $bc$-system, which carries an action of $V^1(\mathfrak{gl}_n)$, and $k$ and $l$ are related by $(k + 1) (l + n + 1) = 1$. This conjecture is known in the case $n=1$, which is somewhat degenerate, and we shall prove it in the first nontrivial case $n=2$. As a consequence, we show that the simple quotient ${\mathcal W}_k(\mathfrak{sl}(3|2))$ is lisse and rational for all positive integers $l>1$. These are new examples of rational ${\mathcal W}$-superalgebras.