code: def _Rforce(self,R,z,phi=0.,t=0.,_forceFloatEval=False):
          
          if not _forceFloatEval and not self.integerSelf == None:
              return self.integerSelf._Rforce(R,z,phi=phi,t=t)
          else:
              r= numpy.sqrt(R**2.+z**2.)
              return -R/r**self.alpha*self.a**(self.alpha-3.)/(3.-self.alpha)\
                  *special.hyp2f1(3.-self.alpha,
                                  self.beta-self.alpha,
                                  4.-self.alpha,
                                  -r/self.a)
docs: NAME:
             _Rforce
          PURPOSE:
             evaluate the radial force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the radial force
          HISTORY:
             2010-07-09 - Written - Bovy (NYU)