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dtrtri

 NAME
      DTRTRI - compute the inverse of a real upper or lower tri-
      angular 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 tri-
      angular 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 tri-
              angular matrix is singular and its inverse can not
              be computed.