NAME

dtptri - compute the inverse of a real upper or lower triangular matrix A stored in packed format

SYNOPSIS

  SUBROUTINE DTPTRI( UPLO, DIAG, N, A, INFO)
  CHARACTER * 1 UPLO, DIAG
  INTEGER N, INFO
  DOUBLE PRECISION A(*)

  SUBROUTINE DTPTRI_64( UPLO, DIAG, N, A, INFO)
  CHARACTER * 1 UPLO, DIAG
  INTEGER*8 N, INFO
  DOUBLE PRECISION A(*)

  SUBROUTINE TPTRI( UPLO, DIAG, N, A, [INFO])
  CHARACTER(LEN=1) :: UPLO, DIAG
  INTEGER :: N, INFO
  REAL(8), DIMENSION(:) :: A

  SUBROUTINE TPTRI_64( UPLO, DIAG, N, A, [INFO])
  CHARACTER(LEN=1) :: UPLO, DIAG
  INTEGER(8) :: N, INFO
  REAL(8), DIMENSION(:) :: A

#include <sunperf.h>

void dtptri(char uplo, char diag, int n, double *a, int *info);

void dtptri_64(char uplo, char diag, long n, double *a, long *info);

dtptri computes the inverse of a real upper or lower triangular matrix A stored in packed format.

UPLO (input)

 = 'U':  A is upper triangular;

 = 'L':  A is lower triangular.

DIAG (input)

 = 'N':  A is non-unit triangular;

 = 'U':  A is unit triangular.

N (input)
The order of the matrix A. N > = 0.
A (input/output)
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 A as follows: if UPLO = 'U', A(i + (j-1)*j/2) = A(i,j) for 1 < =i < =j; if UPLO = 'L', A(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)

 = 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.

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

            A(JC+I-1)  = A(I,J)              A(JC+I-J)  = A(I,J)
    1    CONTINUE                    1    CONTINUE

         JC  = JC + J                      JC  = JC + N - J + 1
    2 CONTINUE                       2 CONTINUE