strti2.f

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      SUBROUTINE strti2( UPLO, DIAG, N, A, LDA, INFO )
*
*  -- LAPACK routine (version 3.2) --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2006
*
*     .. Scalar Arguments ..
      CHARACTER          diag, uplo
      INTEGER            info, lda, n
*     ..
*     .. Array Arguments ..
      REAL               a( lda, * )
*     ..
*
*  Purpose
*  =======
*
*  STRTI2 computes the inverse of a real upper or lower triangular
*  matrix.
*
*  This is the Level 2 BLAS version of the algorithm.
*
*  Arguments
*  =========
*
*  UPLO    (input) CHARACTER*1
*          Specifies whether the matrix A is upper or lower triangular.
*          = 'U':  Upper triangular
*          = 'L':  Lower triangular
*
*  DIAG    (input) CHARACTER*1
*          Specifies whether or not the matrix A is unit triangular.
*          = 'N':  Non-unit triangular
*          = 'U':  Unit triangular
*
*  N       (input) INTEGER
*          The order of the matrix A.  N >= 0.
*
*  A       (input/output) REAL 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 = -k, the k-th argument had an illegal value
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               one
      parameter( one = 1.0e+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            nounit, upper
      INTEGER            j
      REAL               ajj
*     ..
*     .. External Functions ..
      LOGICAL            lsame
      EXTERNAL           lsame
*     ..
*     .. External Subroutines ..
      EXTERNAL           sscal, strmv, xerbla
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          max
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      info = 0
      upper = lsame( uplo, 'U' )
      nounit = lsame( diag, 'N' )
      IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
         info = -1
      ELSE IF( .NOT.nounit .AND. .NOT.lsame( diag, 'U' ) ) THEN
         info = -2
      ELSE IF( n.LT.0 ) THEN
         info = -3
      ELSE IF( lda.LT.max( 1, n ) ) THEN
         info = -5
      END IF
      IF( info.NE.0 ) THEN
         CALL xerbla( 'STRTI2', -info )
         return
      END IF
*
      IF( upper ) THEN
*
*        Compute inverse of upper triangular matrix.
*
         DO 10 j = 1, n
            IF( nounit ) THEN
               a( j, j ) = one / a( j, j )
               ajj = -a( j, j )
            ELSE
               ajj = -one
            END IF
*
*           Compute elements 1:j-1 of j-th column.
*
            CALL strmv( 'Upper', 'No transpose', diag, j-1, a, lda,
     $                  a( 1, j ), 1 )
            CALL sscal( j-1, ajj, a( 1, j ), 1 )
   10    continue
      ELSE
*
*        Compute inverse of lower triangular matrix.
*
         DO 20 j = n, 1, -1
            IF( nounit ) THEN
               a( j, j ) = one / a( j, j )
               ajj = -a( j, j )
            ELSE
               ajj = -one
            END IF
            IF( j.LT.n ) THEN
*
*              Compute elements j+1:n of j-th column.
*
               CALL strmv( 'Lower', 'No transpose', diag, n-j,
     $                     a( j+1, j+1 ), lda, a( j+1, j ), 1 )
               CALL sscal( n-j, ajj, a( j+1, j ), 1 )
            END IF
   20    continue
      END IF
*
      return
*
*     End of STRTI2
*
      END