does anyone know how I can simplify this summation \begin{multline}\sum_{k=1}^{K}\sum_{l=0}^{L-1}\exp\left(\frac{-j3\pi((n-(u-1)L)_N)^2}{N}\right)\exp\left(\frac{j3\pi((n-2-(u-1)L)_N)^2}{N}\right)\\\exp\left(\frac{j3\pi((-l+n-(k-1)L)_N)^2}{N}\right)\exp\left(\frac{-j3\pi((-l+n-2-(k-1)L)_N)^2}{N}\right)\end{multline}
$u$ is an integer between $1$ and $K$ and $n$ is an integer between $0$ and $N-1$. $(x)_N$ is the modulo function with divisor $N$.
Thanks