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NAME
     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( * )



     #include <sunperf.h>

     void stptri(char uplo, char diag, int  n,  float  *sap,  int
               *info) ;

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':

           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

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