So I’ve been trying to wrap my head around this particular piece of information:

I understand what h_l^T h_l and h_c^T h_c do and how to apply the resulting matrices. However, I do not understand how the author arrived at the ‘unseparable’ matrices in (3.6). Reading the references paper [32] did not help too much.

Is anyone here who can clear up the confusion, and possibly show me how this could be extended to even larger filters (just for fun)?