Simultaneous equations and inequalities

Let $\lambda_i, \mu_j$ be non-zero real numbers not all of the same sign and let $a_i, b_k$ be non-zero integers not all of the same sign. We investigate a mixed Diophantine system of the shape \begin{equation*} \begin{cases} \left| \lambda_1 x_1^\theta + \cdots + \lambda_\ell x_\ell^\theta + \mu_1 y_1^\theta + \cdots + \mu_m y_m^\theta \right| < \tau \\[10pt] a_1 x_1^d + \cdots a_\ell x_\ell^d + b_1 z_1^d + \cdots + b_n z_n^d =0, \end{cases} \end{equation*} where $d\geq 2$ is an integer, $\theta > d+1$ is real and non-integral and $\tau$ is a positive real number. For such systems we obtain an asymptotic formula for the number of positive integer solutions $(\textbf{x}, \textbf{y}, \textbf{z}) = (x_1, \ldots, z_n)$ inside a bounded box. Our approach makes use of a two-dimensional version of the classical Hardy-Littlewood circle method and the Davenport--Heilbronn--Freeman method. The proof involves a combination of essentially optimal mean value estimates for the auxiliary exponential sums, together with estimates stemming from the classical Weyl and Weyl-van der Corput inequalities.