likewise, a "subdiagonal" entry is one that is directly below and to the left of the main diagonal, that is, an entry formula_10 with formula_18. general matrix diagonals can be specified by an index formula_19 measured relative to the main diagonal: the main diagonal has formula_20; the superdiagonal has formula_21; the subdiagonal has formula_22; and in general, the formula_19-diagonal consists of the entries formula_10 with formula_25.