DTRTRI (3lapack)

compute the inverse of a real upper or lower triangular matrix A

SYNOPSIS

  SUBROUTINE DTRTRI( UPLO, DIAG, N, A, LDA, INFO )

      CHARACTER      DIAG, UPLO

      INTEGER        INFO, LDA, N

      DOUBLE         PRECISION A( LDA, * )

PURPOSE

  DTRTRI computes the inverse of a real upper or lower triangular matrix A.

  This is the Level 3 BLAS version of the algorithm.

ARGUMENTS

  UPLO    (input) CHARACTER*1
          = 'U':  A is upper triangular;
          = 'L':  A is lower triangular.

  DIAG    (input) CHARACTER*1
          = 'N':  A is non-unit triangular;
          = 'U':  A is unit triangular.

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

  A       (input/output) DOUBLE PRECISION array, dimension (LDA,N)
          On entry, the triangular matrix A.  If UPLO = 'U', the leading N-
          by-N upper triangular part of the array A contains the upper
          triangular matrix, and the strictly lower triangular part of A is
          not referenced.  If UPLO = 'L', the leading N-by-N lower triangular
          part of the array A contains the lower triangular matrix, and the
          strictly upper triangular part of A is not referenced.  If DIAG =
          'U', the diagonal elements of A are also not referenced and are
          assumed to be 1.  On exit, the (triangular) inverse of the original
          matrix, in the same storage format.

  LDA     (input) INTEGER
          The leading dimension of the array A.  LDA >= max(1,N).

  INFO    (output) INTEGER
          = 0: successful exit
          < 0: if INFO = -i, the i-th argument had an illegal value
          > 0: if INFO = i, A(i,i) is exactly zero.  The triangular matrix is
          singular and its inverse can not be computed.