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2. see, e.g., C.J. Pethick and D. Pines, Proc. 4th Int. Conf. on Recent Progress in Many-Body Theories, P. Siemens and R.A. Smith, eds., Springer, Berlin, in print.
3. A.J. Leggett, J.Magn.Magn.Mat. 63&64:406 (1987).
4. P.W. Anderson, Phys.Rev.B 30:4000 (1984).
5. T.M. Rice, K. Ueda, and H.R. Ott, J.Magn.Magn.Mat. 54–57:317 (1986).