STPTRI - compute the inverse of a real upper or lower tri­
       angular matrix A stored in packed format


SYNOPSIS

       SUBROUTINE STPTRI( UPLO, DIAG, N, AP, INFO )

           CHARACTER      DIAG, UPLO

           INTEGER        INFO, N

           REAL           AP( * )


PURPOSE

       STPTRI computes the inverse of a real upper or lower  tri­
       angular 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  for­
               mat.

       INFO    (output) INTEGER
               = 0:  successful exit
               < 0:  if INFO = -i, the i-th argument had an ille­
               gal 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':

             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


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