SUBROUTINE STRTI2F( UPLO, DIAG, N, A, LDA, INFO )

*

*  -- LAPACK routine (version 3.1) --

*     Univ. of Tennessee, Univ. of California Berkeley 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