# Spectral Networks and Deep Locally Connected Networks on Graphs

I’m reading the paper Spectral Networks and Deep Locally Connected Networks on Graphs and I’m having a hard time understanding the notation shown in the picture below (the scribbles are mine):

Specifically, I don’t understand the notation for the matrix F. Why does it include an i and a j?

k is a fixed parameter indicating the input layer and i,j refer to the index of the matrix F , used to traverse throught the matrix.
• So I thought about that too, but then the summation goes up to $f_{k-1}$ (I.e the rows go up to this) yet they say that the matrix is $d_{k-1}$ by $d_{k-1}$. Jan 19 '21 at 8:48