A solution to certain polynomial equations with applications to nonlinear fitting

We present a combinatorial method for solving a certain system of polynomial equations of Vandermonde type in $2N$ variables by reducing it to the problem of solving two special linear systems of size $N$ and rooting a single univariate polynomial of degree $N$. Over $\mathbb{C}$, all solutions can be found with fixed precision using, up to polylogarithmic factors, $O(N^2)$bitwise operations in the worst case. Furthermore, if the data is well conditioned, then this can be reduced to $O(N)$ bit operations, up to polylogarithmic factors. As an application, we show how this can be used to fit data to a complex exponential sum with $N$ terms in the same, nearly optimal, time.