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NAME
     dpptrf - compute the Cholesky factorization of a  real  sym-
     metric positive definite matrix A stored in packed format

SYNOPSIS
     SUBROUTINE DPPTRF( UPLO, N, AP, INFO )

     CHARACTER UPLO

     INTEGER INFO, N

     DOUBLE PRECISION AP( * )



     #include <sunperf.h>

     void dpptrf(char uplo, int n, double *dap, int *info);

PURPOSE
     DPPTRF computes the Cholesky factorization of  a  real  sym-
     metric positive definite matrix A stored in packed format.

     The factorization has the form
        A = U**T * U,  if UPLO = 'U', or
        A = L  * L**T,  if UPLO = 'L',
     where U is an upper triangular matrix and L  is  lower  tri-
     angular.


ARGUMENTS
     UPLO      (input) CHARACTER*1
               = 'U':  Upper triangle of A is stored;
               = 'L':  Lower triangle of A is stored.

     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 triangle of the  sym-
               metric  matrix  A,  packed  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)*(2n-j)/2) = A(i,j) for j<=i<=n.  See below
               for further details.

               On exit, if INFO = 0, the triangular factor U or L
               from  the Cholesky factorization A = U**T*U or A =
               L*L**T, in the same storage format as A.

     INFO      (output) INTEGER
               = 0:  successful exit
               < 0:  if INFO = -i, the i-th argument had an ille-
               gal value
               > 0:  if INFO = i, the leading minor of order i is
               not positive definite, and the factorization could
               not be completed.

FURTHER DETAILS
     The packed storage scheme is illustrated  by  the  following
     example when N = 4, UPLO = 'U':

     Two-dimensional storage of the symmetric matrix A:

        a11 a12 a13 a14
            a22 a23 a24
                a33 a34     (aij = aji)
                    a44

     Packed storage of the upper triangle of A:

     AP = [ a11, a12, a22, a13, a23, a33, a14, a24, a34, a44 ]

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