We introduce a new method of replacing spin operators with pseudofermions. For a spin value S there are 2S pseudofermions each of which can have spin ‘‘up’’ or ‘‘down’’. In contrast to the standard Abrikosov method which has 2S+1 inequivalent fermions, i.e., one for each Sz value, the present pairs are all equivalent. It follows that the general spin expression is obtained by multiplying the S=(1/2 result by a ceratin combinational factor. This aids in the construction of rotationally invariant solutions for the general-spin Kondo problem for the case when n≤2S, where n is the number of conduction-electron scattering channels.
Barnes, S. E., "Alternative Feynman-Diagram Method for the General-Spin Kondo Problem" (1988). Physics Articles and Papers. 66.