code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          m= 4.*R*self.a/((R+self.a)**2+z**2)
          return -2.*self.a/R/nu.sqrt((R+self.a)**2+z**2)\
              *(m*(R**2-self.a2-z**2)/4./(1.-m)/self.a/R*special.ellipe(m)
                +special.ellipk(m))
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:
             2018-08-04 - Written - Bovy (UofT)