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Journal of the Physical Society of Japan 68 (1999) pp. 2361–2365 Bosonic Quantum Spherical Model for the XY Magnet Yu-qiang Ma Progress of Theoretical Physics Vol. 43 No. 5 (1970) pp. 1148–1169 A Phenomenological Model of Cooperativity N. E. Frankel
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Matsubara et al. (1956) studied this question.
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