This will give you the index of the largest element in matrix F.
ind = np.unravel_index(np.argmax(F),F.shape)
This will give you the row number of the largest element in column j.
max_row_in_col_j = np.argmax(F[:,j])
This will give you the column number of the largest element in row i.
max_col_in_row_i = np.argmax(F[i,:])