stptri(3)

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( * )

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 ma

trix, in the same packed storage format.

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 comput

ed.

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

LAPACK version 3.0 15 June 2000

STPTRI(3)