stptri (l) - Linux Manuals

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

NAME

STPTRI - computes 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

= aqUaq: A is upper triangular;
= aqLaq: A is lower triangular.

DIAG (input) CHARACTER*1

= aqNaq: A is non-unit triangular;
= aqUaq: 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 = aqUaq, AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j; if UPLO = aqLaq, 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 = aqUaq: UPLO = aqLaq:

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