      SUBROUTINE DPPTRF( UPLO, N, AP, INFO )
*
*  -- LAPACK routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     .. Scalar Arguments ..
      CHARACTER          UPLO
      INTEGER            INFO, N
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   AP( * )
*     ..
*
*  Purpose
*  =======
*
*  DPPTRF computes the Cholesky factorization of a real symmetric
*  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 triangular.
*
*  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 symmetric 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 illegal 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 ]
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   ONE, ZERO
      PARAMETER          ( ONE = 1.0D+0, ZERO = 0.0D+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            UPPER
      INTEGER            J, JC, JJ
      DOUBLE PRECISION   AJJ
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      DOUBLE PRECISION   DDOT
      EXTERNAL           LSAME, DDOT
*     ..
*     .. External Subroutines ..
      EXTERNAL           DSCAL, DSPR, DTPSV, XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          SQRT
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      INFO = 0
      UPPER = LSAME( UPLO, 'U' )
      IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
         INFO = -1
      ELSE IF( N.LT.0 ) THEN
         INFO = -2
      END IF
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'DPPTRF', -INFO )
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( N.EQ.0 )
     $   RETURN
*
      IF( UPPER ) THEN
*
*        Compute the Cholesky factorization A = U'*U.
*
         JJ = 0
         DO 10 J = 1, N
            JC = JJ + 1
            JJ = JJ + J
*
*           Compute elements 1:J-1 of column J.
*
            IF( J.GT.1 )
     $         CALL DTPSV( 'Upper', 'Transpose', 'Non-unit', J-1, AP,
     $                     AP( JC ), 1 )
*
*           Compute U(J,J) and test for non-positive-definiteness.
*
            AJJ = AP( JJ ) - DDOT( J-1, AP( JC ), 1, AP( JC ), 1 )
            IF( AJJ.LE.ZERO ) THEN
               AP( JJ ) = AJJ
               GO TO 30
            END IF
            AP( JJ ) = SQRT( AJJ )
   10    CONTINUE
      ELSE
*
*        Compute the Cholesky factorization A = L*L'.
*
         JJ = 1
         DO 20 J = 1, N
*
*           Compute L(J,J) and test for non-positive-definiteness.
*
            AJJ = AP( JJ )
            IF( AJJ.LE.ZERO ) THEN
               AP( JJ ) = AJJ
               GO TO 30
            END IF
            AJJ = SQRT( AJJ )
            AP( JJ ) = AJJ
*
*           Compute elements J+1:N of column J and update the trailing
*           submatrix.
*
            IF( J.LT.N ) THEN
               CALL DSCAL( N-J, ONE / AJJ, AP( JJ+1 ), 1 )
               CALL DSPR( 'Lower', N-J, -ONE, AP( JJ+1 ), 1,
     $                    AP( JJ+N-J+1 ) )
               JJ = JJ + N - J + 1
            END IF
   20    CONTINUE
      END IF
      GO TO 40
*
   30 CONTINUE
      INFO = J
*
   40 CONTINUE
      RETURN
*
*     End of DPPTRF
*
      END
