CPBTRF(3f)

NAME
CPBTRF - the Cholesky factorization of a complex Hermitian positive definite band matrix A

SYNOPSIS

SUBROUTINE CPBTRF(
    UPLO, N, KD, AB, LDAB, INFO )

CHARACTER
    UPLO

INTEGER
    INFO, KD, LDAB, N

COMPLEX
    AB( LDAB, * )

PURPOSE
CPBTRF computes the Cholesky factorization of a complex Hermitian positive definite band matrix A. The factorization has the form
   A = U**H * U, if UPLO = 'U', or
   A = L * L**H, if UPLO = 'L',
where U is an upper triangular matrix and L is lower triangular.

ARGUMENTS

UPLO    (input) CHARACTER*1
    = 'U': Upper triangle of A is stored;
    = 'L': Lower triangle of A is stored.

N       (input) INTEGER
    The order of the matrix A. N >= 0.

KD      (input) INTEGER
    The number of superdiagonals of the matrix A if UPLO = 'U', or the number of subdiagonals if UPLO = 'L'. KD >= 0.

AB      (input/output) COMPLEX array, dimension (LDAB,N)
    On entry, the upper or lower triangle of the Hermitian band matrix A, stored in the first KD+1 rows of the array. The j-th column of A is stored in the j-th column of the array AB as follows: if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j; if UPLO = 'L', AB(1+i-j,j) = A(i,j) for j<=i<=min(n,j+kd). On exit, if INFO = 0, the triangular factor U or L from the Cholesky factorization A = U**H*U or A = L*L**H of the band matrix A, in the same storage format as A.

LDAB    (input) INTEGER
    The leading dimension of the array AB. LDAB >= KD+1.

INFO    (output) INTEGER
    = 0: successful exit
    < 0: if INFO = -i, the i-th argument had an illegal value
    > 0: if INFO = i, the leading minor of order i is not positive definite, and the factorization could not be completed.

FURTHER DETAILS
The band storage scheme is illustrated by the following example, when N = 6, KD = 2, and UPLO = 'U':
On entry: On exit:
    * * a13 a24 a35 a46 * * u13 u24 u35 u46
    * a12 a23 a34 a45 a56 * u12 u23 u34 u45 u56
   a11 a22 a33 a44 a55 a66 u11 u22 u33 u44 u55 u66 Similarly, if UPLO = 'L' the format of A is as follows:
On entry: On exit:
   a11 a22 a33 a44 a55 a66 l11 l22 l33 l44 l55 l66
   a21 a32 a43 a54 a65 * l21 l32 l43 l54 l65 *
   a31 a42 a53 a64 * * l31 l42 l53 l64 * * Array elements marked * are not used by the routine.
Contributed by
Peter Mayes and Giuseppe Radicati, IBM ECSEC, Rome, March 23, 1989