dtptri man page on IRIX

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DTPTRI(3F)							    DTPTRI(3F)

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

SYNOPSIS
     SUBROUTINE DTPTRI( UPLO, DIAG, N, AP, INFO )

	 CHARACTER	DIAG, UPLO

	 INTEGER	INFO, N

	 DOUBLE		PRECISION AP( * )

PURPOSE
     DTPTRI 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) DOUBLE PRECISION 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

									Page 1

DTPTRI(3F)							    DTPTRI(3F)

	   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

									Page 2

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