My question concerns the answer on this post: Characterisation of the rate distortion function: issue with functional derivative
What do the symbols $\delta_{x',x_0}$ and $\delta_{\hat{x}_0,\hat{x}}$ mean; in particular, how is the third line derived from the second one? Does anyone have a reference for these definitions/notation? The Wikipedia article on functional derivatives doesn't seem to have them...Thank you. (Sorry I couldn't comment on the post because I don't have enough reputation.)