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The definition of an alternant system is given in terms of commutation and anticommutation relations of the Hamiltonian with certain operators. Necessary and sufficient conditions that the Hamiltonian fulfill these relationships are formulated. Different known cases of alternant systems are derived from the definition. Pairing properties are shown to follow as consequences of the definition, and they are summarized in simple general equations for matrix elements of an arbitrary operator.
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Jaroslav Koutecký (1966) studied this question.
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