The fact that we only have an integer or half-integer as the real part of is strongly suggested from the property of . Let us summarize the argument. We first convert to as , where is an arbitrary integer and is an arbitrary complex number. We note that for an arbitrary integer ,
It becomes possible to determine in the wide range of by allowing . The MST formalism is now very useful in the fully numerical evaluation of homogeneous solutions of the Teukolsky equation. As a first step, Fujita, Hikida and Tagoshi  considered generic bound geodesic orbits around a Kerr black hole and evaluate the energy and angular momentum flux to infinity as well as the rate of change of the Carter constant in a wide range of orbital parameters.
The critical value of when becomes complex is not very small. The complex does not appear in the analytic evaluation of in the low frequency expansion in powers of . Thus, at the first glance, it seems impossible to express the complex in the power series expansion of . However, Hikida et al.  pointed out that it is possible to evaluate very accurately in terms of the power series expansion of , even if is larger than a critical value and is complex. Such an analytical expression of is very useful in the numerical root finding of Equation (174) as well as in the analytical calculation of the homogeneous solutions.
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