A square matrix with nonzero elements only on the diagonal and slots horizontally or vertically adjacent the diagonal (i.e., along the subdiagonal and superdiagonal), [a_11 | a_12 | 0 | 0 | ... | 0 | 0 a_21 | a_22 | a_23 | ⋱ | ⋱ | 0 | 0 0 | a_32 | a_33 | ⋱ | ⋱ | a_(n - 2, n - 1) | 0 ⋮ | ⋱ | ⋱ | ⋱ | ⋱ | a_(n - 1, n - 1) | a_(n - 1, n) 0 | 0 | ... | ... | ... | a_(n, n - 1) | a_(n n)].

diagonal matrix | Jacobi method | subdiagonal | superdiagonal