SYNOPSIS

SUBROUTINE DPPTRI(

UPLO, N, AP, INFO )

    

CHARACTER UPLO

    

INTEGER INFO, N

    

DOUBLE PRECISION AP( * )

PURPOSE

DPPTRI computes the inverse of a real symmetric positive definite matrix A using the Cholesky factorization A = U**T*U or A = L*L**T computed by DPPTRF.

ARGUMENTS

UPLO (input) CHARACTER*1

= 'U': Upper triangular factor is stored in AP;
= 'L': Lower triangular factor is stored in AP.

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 triangular factor U or L from the Cholesky factorization A = U**T*U or A = L*L**T, packed columnwise as a linear array. The j-th column of U or L is stored in the array AP as follows: if UPLO = 'U', AP(i + (j-1)*j/2) = U(i,j) for 1<=i<=j; if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = L(i,j) for j<=i<=n. On exit, the upper or lower triangle of the (symmetric) inverse of A, overwriting the input factor U or L.

INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
> 0: if INFO = i, the (i,i) element of the factor U or L is zero, and the inverse could not be computed.