Degenerate Perturbation Theory in Quantum Mechanics

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Abstract
The relations are discussed among a number of different formulations of perturbation theory for an effective Hamiltonian of a general quantum system. Although the definitions and the formulations are apparently different, it is shown that the effective Hamiltonians, which have appeared in the history of time-independent approaches, can be written simply in a unified form. Expansion formulae are given explicitly for both the non-Hermitian and Hermitian effective Hamiltonians, and a discussion is given on a method for summing up the expansion series and obtaining a convergent result.