Quadratic equations in metabelian Baumslag-Solitar groups

For a finitely generated group $G$, the \emph{Diophantine problem} over $G$ is the algorithmic problem of deciding whether a given equation $W(z_1,z_2,\dots ,z_k)=1$ (perhaps restricted to a fixed subclass of equations) has a solution in $G$. In this paper, we investigate the algorithmic complexity of the Diophantine problem for the class $\mathcal{C}$ of quadratic equations over the metabelian Baumslag-Solitar groups $\mathbf{B}\mathbf{S}(1,n)$. We prove that this problem is $\mathbf{N}\mathbf{P}$-complete whenever $n\mathrm{\ne }\pm 1$, and determine the algorithmic complexity for various subclasses (orientable, nonorientable etc.) of $\mathcal{C}$.