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2. Suits, C. G., Phys. Rev., 55, 561 (1939).
3. Malter, L., Johnson, E. O., and Webster, W. M., R.C.A. Rev., 12, 415 (1951).
4. Donahue, T., and Dicke, G. H., Phys. Rev., 81, 248 (1951).
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