def vatu0(self,E,Lz,u0,R,retv2=False):
        """
        NAME:
           vatu0
        PURPOSE:
           calculate the velocity at u0
        INPUT:
           E - energy
           Lz - angular momentum
           u0 - u0
           R - radius corresponding to u0,pi/2.
           retv2= (False), if True return v^2
        OUTPUT:
           velocity
        HISTORY:
           2012-11-29 - Written - Bovy (IAS)
        """                        
        v2= (2.*(E-actionAngleStaeckel.potentialStaeckel(u0,numpy.pi/2.,
                                                         self._pot,
                                                         self._delta))
             -Lz**2./R**2.)
        if retv2: return v2
        v2[(v2 < 0.)*(v2 > -10.**-7.)]= 0.
        return numpy.sqrt(v2)
NAME:
           vatu0
        PURPOSE:
           calculate the velocity at u0
        INPUT:
           E - energy
           Lz - angular momentum
           u0 - u0
           R - radius corresponding to u0,pi/2.
           retv2= (False), if True return v^2
        OUTPUT:
           velocity
        HISTORY:
           2012-11-29 - Written - Bovy (IAS)