How to transform the following double summation to matrix notation?
$$\sum_{\substack{i}}\sum_{\substack{j}} f(\theta_{i})f(\theta_{j})\sin(|\theta_{i} - \theta_{j}|)$$
This answer Matrix notion for a double summation gives me some idea, but $\sin$ is different with $\cos$, and I also don't know how to deal with the absolute value.
