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.