def _Rzderiv(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _Rzderiv
        PURPOSE:
           evaluate the mixed R,z derivative for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           d2phi/dR/dz
        HISTORY:
           2013-08-28 - Written - Bovy (IAS)
        """
        sqrtbz= nu.sqrt(self._b2+z**2.)
        asqrtbz= self._a+sqrtbz
        if isinstance(R,float) and sqrtbz == asqrtbz:
            return -(3.*R*z/(R**2.+asqrtbz**2.)**2.5)
        else:
            return -(3.*R*z*asqrtbz
                     /sqrtbz/(R**2.+asqrtbz**2.)**2.5)
def _Rzderiv ( self , R , z , phi = 0. , t = 0. ) : sqrtbz = nu . sqrt ( self . _b2 + z ** 2. ) asqrtbz = self . _a + sqrtbz if isinstance ( R , float ) and sqrtbz == asqrtbz : return - ( 3. * R * z / ( R ** 2. + asqrtbz ** 2. ) ** 2.5 ) else : return - ( 3. * R * z * asqrtbz / sqrtbz / ( R ** 2. + asqrtbz ** 2. ) ** 2.5 )
NAME:
           _Rzderiv
        PURPOSE:
           evaluate the mixed R,z derivative for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           d2phi/dR/dz
        HISTORY:
           2013-08-28 - Written - Bovy (IAS)