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We present a comprehensive theory that reduces the total power of products of spin operators. This theory improves the previous one [P.J. Jensen, F. Aguilera-Granja, Phys. Lett. A 269 (2000) 158] in two aspects. One is that for the set of spin operators S+, S−, Sz, a new method is suggested where the expansion coefficients in the reduction formula can be solved from linear equations. This new method...
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