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SUBROUTINE RK2

PURPOSE
   INTEGRATES A FIRST ORDER DIFFERENTIAL EQUATION
   DY/DX=FUN(X,Y) AND PRODUCES A TABLE OF INTEGRATED VALUES

USAGE
   CALL RK2(FUN,H,XI,YI,K,N,VEC)

DESCRIPTION OF PARAMETERS
   FUN-USER-SUPPLIED FUNCTION SUBPROGRAM WITH ARGUMENTS X,Y
       WHICH GIVES DY/DX
   H  -STEP SIZE
   XI -INITIAL VALUE OF X
   YI -INITIAL VALUE OF Y WHERE YI=Y(XI)
   K  -THE INTERVAL AT WHICH COMPUTED VALUES ARE TO BE STORED
   N  -THE NUMBER OF VALUES TO BE STORED
   VEC-THE RESULTANT VECTOR OF LENGTH N IN WHICH COMPUTED
       VALUES OF Y ARE TO BE STORED

REMARKS
   NONE

SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
   FUN - USER-SUPPLIED FUNCTION SUBPROGRAM FOR DY/DX
   CALLING PROGRAM MUST HAVE FORTRAN EXTERNAL STATEMENT
   CONTAINING NAMES OF FUNCTION SUBPROGRAMS LISTED IN CALL TO
   RK2

METHOD
   FOURTH ORDER RUNGE-KUTTA INTEGRATION ON A RECURSIVE BASIS AS
   SHOWN IN F.B. HILDEBRAND, 'INTRODUCTION TO NUMERICAL
   ANALYSIS', MCGRAW-HILL, NEW YORK, 1956