The superdiagonal of a square matrix is the set of elements directly above the elements comprising the diagonal. For example, in the following matrix, the diagonal elements are denoted d_i and the superdiagonal elements are denoted s_i, [d_1 | s_1 | a_13 | ... | a_(1n) a_21 | d_2 | s_2 | ⋱ | a_(2n) a_31 | a_32 | d_3 | ⋱ | a_(3n) ⋮ | ⋱ | ⋱ | ⋱ | ⋱ a_(n1) | a_(n2) | a_(n3) | ... | d_n].

diagonal | subdiagonal | tridiagonal matrix