
STPTRI(l) ) STPTRI(l)
NAME
STPTRI  compute the inverse of a real upper or lower triangular 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 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 nonunit 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 jth column of A is stored in the array AP as follows: if UPLO = 'U',
AP(i + (j1)*j/2) = A(i,j) for 1<=i<=j; if UPLO = 'L', AP(i + (j1)*((2*nj)/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 ith 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 pro
gram 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+I1) = A(I,J) AP(JC+IJ) = A(I,J)
1 CONTINUE 1 CONTINUE
JC = JC + J JC = JC + N  J + 1
2 CONTINUE 2 CONTINUE
LAPACK version 3.0 15 June 2000 STPTRI(l) 
