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SUBROUTINE DQG12

PURPOSE
   TO COMPUTE INTEGRAL(FCT(X), SUMMED OVER X FROM XL TO XU)

USAGE
   CALL DQG12 (XL,XU,FCT,Y)
   PARAMETER FCT REQUIRES AN EXTERNAL STATEMENT

DESCRIPTION OF PARAMETERS
   XL	  - DOUBLE PRECISION LOWER BOUND OF THE INTERVAL.
   XU	  - DOUBLE PRECISION UPPER BOUND OF THE INTERVAL.
   FCT	  - THE NAME OF AN EXTERNAL DOUBLE PRECISION FUNCTION
	    SUBPROGRAM USED.
   Y	  - THE RESULTING DOUBLE PRECISION INTEGRAL VALUE.

REMARKS
   NONE

SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
   THE EXTERNAL DOUBLE PRECISION FUNCTION SUBPROGRAM FCT(X)
   MUST BE FURNISHED BY THE USER.

METHOD
   EVALUATION IS DONE BY MEANS OF 12-POINT GAUSS QUADRATURE
   FORMULA, WHICH INTEGRATES POLYNOMIALS UP TO DEGREE 23
   EXACTLY. FOR REFERENCE, SEE
   V.I.KRYLOV, APPROXIMATE CALCULATION OF INTEGRALS,
   MACMILLAN, NEW YORK/LONDON, 1962, PP.100-111 AND 337-340.