pyhispen.f

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C***********************************************************************    
    
      FUNCTION pyhispen(XREIN,XIMIN,IREIM)    
    
C...Calculates real and imaginary part of Spence function; see  
C...G. 't Hooft and M. Veltman, Nucl. Phys. B153 (1979) 365.    
      common/ludat1/mstu(200),paru(200),mstj(200),parj(200) 
      SAVE /ludat1/ 
      dimension b(0:14) 
    
      DATA b/   
     & 1.000000e+00,        -5.000000e-01,         1.666667e-01,    
     & 0.000000e+00,        -3.333333e-02,         0.000000e+00,    
     & 2.380952e-02,         0.000000e+00,        -3.333333e-02,    
     & 0.000000e+00,         7.575757e-02,         0.000000e+00,    
     &-2.531135e-01,         0.000000e+00,         1.166667e+00/    
    
      xre=xrein 
      xim=ximin 
      IF(abs(1.-xre).LT.1.e-6.AND.abs(xim).LT.1.e-6) THEN   
        IF(ireim.EQ.1) pyhispen=paru(1)**2/6. 
        IF(ireim.EQ.2) pyhispen=0.    
        RETURN  
      ENDIF 
    
      xmod=sqrt(xre**2+xim**2)  
      IF(xmod.LT.1.e-6) THEN    
        IF(ireim.EQ.1) pyhispen=0.    
        IF(ireim.EQ.2) pyhispen=0.    
        RETURN  
      ENDIF 
    
      xarg=sign(acos(xre/xmod),xim) 
      sp0re=0.  
      sp0im=0.  
      sgn=1.    
      IF(xmod.GT.1.) THEN   
        algxre=log(xmod)    
        algxim=xarg-sign(paru(1),xarg)  
        sp0re=-paru(1)**2/6.-(algxre**2-algxim**2)/2.   
        sp0im=-algxre*algxim    
        sgn=-1. 
        xmod=1./xmod    
        xarg=-xarg  
        xre=xmod*cos(xarg)  
        xim=xmod*sin(xarg)  
      ENDIF 
      IF(xre.GT.0.5) THEN   
        algxre=log(xmod)    
        algxim=xarg 
        xre=1.-xre  
        xim=-xim    
        xmod=sqrt(xre**2+xim**2)    
        xarg=sign(acos(xre/xmod),xim)   
        algyre=log(xmod)    
        algyim=xarg 
        sp0re=sp0re+sgn*(paru(1)**2/6.-(algxre*algyre-algxim*algyim))   
        sp0im=sp0im-sgn*(algxre*algyim+algxim*algyre)   
        sgn=-sgn    
      ENDIF 
    
      xre=1.-xre    
      xim=-xim  
      xmod=sqrt(xre**2+xim**2)  
      xarg=sign(acos(xre/xmod),xim) 
      zre=-log(xmod)    
      zim=-xarg 
    
      spre=0.   
      spim=0.   
      savere=1. 
      saveim=0. 
      DO 100 i=0,14 
      termre=(savere*zre-saveim*zim)/float(i+1) 
      termim=(savere*zim+saveim*zre)/float(i+1) 
      savere=termre 
      saveim=termim 
      spre=spre+b(i)*termre 
  100 spim=spim+b(i)*termim 
    
      IF(ireim.EQ.1) pyhispen=sp0re+sgn*spre  
      IF(ireim.EQ.2) pyhispen=sp0im+sgn*spim  
    
      RETURN    
      END