are the most important special cases. The ×-0 notation is a handy way to describe patterns of zeros and nonzeros in a matrix. Each “×” designates a nonzero scalar.
In general, a matrix A = (aij) has lower bandwidth p if aij = 0 whenever i > j + p. Thus, an upper triangular matrix has lower bandwidth 0 and a tridiagonal matrix has lower bandwidth 1. A matrix A = (aij ) has upper bandwidth q if aij = 0 whenever j > i + q. Thus, a lower triangular matrix has upper bandwidth 0 and a tridiagonal matrix has lower bandwidth 1. Here is a matrix with upper bandwidth 2 and lower bandwidth 3