```      SUBROUTINE STPTRI( UPLO, DIAG, N, AP, 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, N
*     ..
*     .. Array Arguments ..
REAL               AP( * )
*     ..
*
*  Purpose
*  =======
*
*  STPTRI computes the inverse of a real upper or lower triangular
*  matrix A stored in packed format.
*
*  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.
*
*  AP      (input/output) REAL array, dimension (N*(N+1)/2)
*          On entry, the upper or lower triangular matrix A, stored
*          columnwise in a linear array.  The j-th column of A is stored
*          in the array AP as follows:
*          if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j;
*          if UPLO = 'L', AP(i + (j-1)*((2*n-j)/2) = A(i,j) for j<=i<=n.
*          See below for further details.
*          On exit, the (triangular) inverse of the original matrix, in
*          the same packed storage format.
*
*  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.
*
*  Further Details
*  ===============
*
*  A triangular matrix A can be transferred to packed storage using one
*  of the following program segments:
*
*  UPLO = 'U':                      UPLO = 'L':
*
*        JC = 1                           JC = 1
*        DO 2 J = 1, N                    DO 2 J = 1, N
*           DO 1 I = 1, J                    DO 1 I = J, N
*              AP(JC+I-1) = A(I,J)              AP(JC+I-J) = A(I,J)
*      1    CONTINUE                    1    CONTINUE
*           JC = JC + J                      JC = JC + N - J + 1
*      2 CONTINUE                       2 CONTINUE
*
*  =====================================================================
*
*     .. Parameters ..
REAL               ONE, ZERO
PARAMETER          ( ONE = 1.0E+0, ZERO = 0.0E+0 )
*     ..
*     .. Local Scalars ..
LOGICAL            NOUNIT, UPPER
INTEGER            J, JC, JCLAST, JJ
REAL               AJJ
*     ..
*     .. External Functions ..
LOGICAL            LSAME
EXTERNAL           LSAME
*     ..
*     .. External Subroutines ..
EXTERNAL           SSCAL, STPMV, XERBLA
*     ..
*     .. 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
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'STPTRI', -INFO )
RETURN
END IF
*
*     Check for singularity if non-unit.
*
IF( NOUNIT ) THEN
IF( UPPER ) THEN
JJ = 0
DO 10 INFO = 1, N
JJ = JJ + INFO
IF( AP( JJ ).EQ.ZERO )
\$            RETURN
10       CONTINUE
ELSE
JJ = 1
DO 20 INFO = 1, N
IF( AP( JJ ).EQ.ZERO )
\$            RETURN
JJ = JJ + N - INFO + 1
20       CONTINUE
END IF
INFO = 0
END IF
*
IF( UPPER ) THEN
*
*        Compute inverse of upper triangular matrix.
*
JC = 1
DO 30 J = 1, N
IF( NOUNIT ) THEN
AP( JC+J-1 ) = ONE / AP( JC+J-1 )
AJJ = -AP( JC+J-1 )
ELSE
AJJ = -ONE
END IF
*
*           Compute elements 1:j-1 of j-th column.
*
CALL STPMV( 'Upper', 'No transpose', DIAG, J-1, AP,
\$                  AP( JC ), 1 )
CALL SSCAL( J-1, AJJ, AP( JC ), 1 )
JC = JC + J
30    CONTINUE
*
ELSE
*
*        Compute inverse of lower triangular matrix.
*
JC = N*( N+1 ) / 2
DO 40 J = N, 1, -1
IF( NOUNIT ) THEN
AP( JC ) = ONE / AP( JC )
AJJ = -AP( JC )
ELSE
AJJ = -ONE
END IF
IF( J.LT.N ) THEN
*
*              Compute elements j+1:n of j-th column.
*
CALL STPMV( 'Lower', 'No transpose', DIAG, N-J,
\$                     AP( JCLAST ), AP( JC+1 ), 1 )
CALL SSCAL( N-J, AJJ, AP( JC+1 ), 1 )
END IF
JCLAST = JC
JC = JC - N + J - 2
40    CONTINUE
END IF
*
RETURN
*
*     End of STPTRI
*
END

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