code: def ISSO_eq_at_pole(r, chi):
      
      return r**3*(r**2*(r-6)+chi**2*(3*r+4))+chi**4*(3*r*(r-2)+chi**2)
docs: Polynomial that enables the calculation of the Kerr polar
      (inclination = +/- pi/2) innermost stable spherical orbit
      (ISSO) radius via its roots.  Physical solutions are
      between 6 and 1+sqrt[3]+sqrt[3+2sqrt[3]].
  
      Parameters
      -----------
      r: float
          the radial coordinate in BH mass units
      chi: float
          the BH dimensionless spin parameter
  
      Returns
      ----------
      float
          r**3*(r**2*(r-6)+chi**2*(3*r+4))+chi**4*(3*r*(r-2)+chi**2)