I am doing a project on group association schemes, in particular looking at the structure constant $$p_{KL}^M = \#\{(x, y, xy) : x \in K, y\in L, xy \in M\}$$ where $K, L$ and $M$ are conjugacy classes.

I have been given a formula to help me, but cannot get hold of it anymore! It has this expression on the right-hand side: $$ \sum_{\chi \in \DeclareMathOperator{\irr}{Irr} \irr(G)} \frac{\chi(x) \chi(y)}{\chi(xy)}$$ Does anyone recognise this?