in the case of a square matrix, the "main" or "principal diagonal" is the diagonal line of entries running from the top-left corner to the bottom-right corner. for a matrix formula_7 with row index specified by formula_8 and column index specified by formula_9, these would be entries formula_10 with formula_11. for example, the identity matrix can be defined as having entries of 1 on the main diagonal and zeroes elsewhere: