pyfcmp.f

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C*********************************************************************
 
C...PYFCMP: Auxiliary to PYPDFU and PYPTIS.
C...Giving the x*f pdf of a companion quark, with its partner at XS,
C...using an approximate gluon density like (1-X)^NPOW/X. The value
C...corresponds to an unrescaled range between 0 and 1-X.
 
      FUNCTION pyfcmp(XC,XS,NPOW)
      IMPLICIT NONE
      DOUBLE PRECISION xc, xs, y, pyfcmp,fac
      INTEGER npow
 
      pyfcmp=0d0
C...Parent gluon momentum fraction
      y=xc+xs
      IF (y.GE.1d0) RETURN
C...Common factor (includes factor XC, since PYFCMP=x*f)
      fac=3d0*xc*xs*(xc**2+xs**2)/(y**4)
C...Store normalized companion x*f distribution.
      IF (npow.LE.0) THEN
        pyfcmp=fac/(2d0-xs*(3d0-xs*(3d0-2d0*xs)))
      ELSEIF (npow.EQ.1) THEN
        pyfcmp=fac*(1d0-y)/(2d0+xs**2*(-3d0+xs)+3d0*xs*log(xs))
      ELSEIF (npow.EQ.2) THEN
        pyfcmp=fac*(1d0-y)**2/(2d0*((1d0-xs)*(1d0+xs*(4d0+xs))
     &       +3d0*xs*(1d0+xs)*log(xs)))
      ELSEIF (npow.EQ.3) THEN
        pyfcmp=fac*(1d0-y)**3*2d0/(4d0+27d0*xs-31d0*xs**3
     &       +6d0*xs*log(xs)*(3d0+2d0*xs*(3d0+xs)))
      ELSEIF (npow.GE.4) THEN
        pyfcmp=fac*(1d0-y)**4/(2d0*(1d0+2d0*xs)*((1d0-xs)*(1d0+
     &       xs*(10d0+xs))+6d0*xs*log(xs)*(1d0+xs)))
      ENDIF
      RETURN
      END