Band Storage

An -by- band matrix  with subdiagonals and superdiagonals may be stored compactly in a two-dimensional array with rows and columns. Columns of the matrix are stored in corresponding columns of the array, and diagonals of the matrix are stored in rows of the array. This storage scheme should be used in practice only if , although LAPACK routines work correctly for all values of and . In LAPACK, arrays that hold matrices in band storage have names ending in `B'.

To be precise, is stored in AB() for . For example, when , and :

The elements marked in the upper left and lower right corners of the array AB need not be set, and are not referenced by LAPACK routines.

Note: when a band matrix is supplied for factorization, space  must be allowed to store an additional superdiagonals, generated by fill-in as a result of row interchanges. This means that the matrix is stored according to the above scheme, but with superdiagonals.

Triangular band matrices are stored in the same format, with either if upper triangular, or if lower triangular.

For symmetric or Hermitian band matrices with subdiagonals or superdiagonals, only the upper or lower triangle (as specified by UPLO) need be stored:

• if UPLO = `U', is stored in AB() for ;

• if UPLO = `L', is stored in AB() for .

For example, when and :

EISPACK  routines use a different storage scheme for band matrices, in which rows of the matrix are stored in corresponding rows of the array, and diagonals of the matrix are stored in columns of the array (see Appendix D).