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DQL4.FOR
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1985-11-29
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53 lines
C
C ..................................................................
C
C SUBROUTINE DQL4
C
C PURPOSE
C TO COMPUTE INTEGRAL(EXP(-X)*FCT(X), SUMMED OVER X
C FROM 0 TO INFINITY).
C
C USAGE
C CALL DQL4 (FCT,Y)
C PARAMETER FCT REQUIRES AN EXTERNAL STATEMENT
C
C DESCRIPTION OF PARAMETERS
C FCT - THE NAME OF AN EXTERNAL DOUBLE PRECISION FUNCTION
C SUBPROGRAM USED.
C Y - THE RESULTING DOUBLE PRECISION INTEGRAL VALUE.
C
C REMARKS
C NONE
C
C SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
C THE EXTERNAL DOUBLE PRECISION FUNCTION SUBPROGRAM FCT(X)
C MUST BE FURNISHED BY THE USER.
C
C METHOD
C EVALUATION IS DONE BY MEANS OF 4-POINT GAUSSIAN-LAGUERRE
C QUADRATURE FORMULA, WHICH INTEGRATES EXACTLY,
C WHENEVER FCT(X) IS A POLYNOMIAL UP TO DEGREE 7.
C FOR REFERENCE, SEE
C SHAO/CHEN/FRANK, TABLES OF ZEROS AND GAUSSIAN WEIGHTS OF
C CERTAIN ASSOCIATED LAGUERRE POLYNOMIALS AND THE RELATED
C GENERALIZED HERMITE POLYNOMIALS, IBM TECHNICAL REPORT
C TR00.1100 (MARCH 1964), PP.24-25.
C
C ..................................................................
C
SUBROUTINE DQL4(FCT,Y)
C
C
DOUBLE PRECISION X,Y,FCT
C
X=.9395070912301133D1
Y=.53929470556132745D-3*FCT(X)
X=.45366202969211280D1
Y=Y+.38887908515005384D-1*FCT(X)
X=.17457611011583466D1
Y=Y+.35741869243779969D0*FCT(X)
X=.32254768961939231D0
Y=Y+.60315410434163360D0*FCT(X)
RETURN
END