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SUBROUTINE DDGT3

PURPOSE
   TO COMPUTE A VECTOR OF DERIVATIVE VALUES GIVEN VECTORS OF
   ARGUMENT VALUES AND CORRESPONDING FUNCTION VALUES.

USAGE
   CALL DDGT3(X,Y,Z,NDIM,IER)

DESCRIPTION OF PARAMETERS
   X	 -  GIVEN VECTOR OF DOUBLE PRECISION ARGUMENT VALUES
	    (DIMENSION NDIM)
   Y	 -  GIVEN VECTOR OF DOUBLE PRECISION FUNCTION VALUES
	    CORRESPONDING TO X (DIMENSION NDIM)
   Z	 -  RESULTING VECTOR OF DOUBLE PRECISION DERIVATIVE
	    VALUES (DIMENSION NDIM)
   NDIM  -  DIMENSION OF VECTORS X,Y AND Z
   IER	 -  RESULTING ERROR PARAMETER
	    IER  = -1  - NDIM IS LESS THAN 3
	    IER  =  0  - NO ERROR
	    IER POSITIVE  - X(IER) = X(IER-1) OR X(IER) =
			    X(IER-2)

REMARKS
   (1)	 IF IER = -1,2,3, THEN THERE IS NO COMPUTATION.
   (2)	 IF IER =  4,...,N, THEN THE DERIVATIVE VALUES Z(1)
	 ,..., Z(IER-1) HAVE BEEN COMPUTED.
   (3)	 Z CAN HAVE THE SAME STORAGE ALLOCATION AS X OR Y.  IF
	 X OR Y IS DISTINCT FROM Z, THEN IT IS NOT DESTROYED.

SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
   NONE

METHOD
   EXCEPT AT THE ENDPOINTS X(1) AND X(NDIM), Z(I) IS THE
   DERIVATIVE AT X(I) OF THE LAGRANGIAN INTERPOLATION
   POLYNOMIAL OF DEGREE 2 RELEVANT TO THE 3 SUCCESSIVE POINTS
   (X(I+K),Y(I+K)) K = -1,0,1. (SEE HILDEBRAND, F.B.,
   INTRODUCTION TO NUMERICAL ANALYSIS, MC GRAW-HILL, NEW YORK/
   TORONTO/LONDON, 1956, PP. 64-68.)