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5. For small\bia, eq. (4) reduces to the result ofV. Weisskopf andE. Wigner:loo. cit. (4), eqs. (156), (21). Forya = yb, and arbitrary\Ma, eq. (4) leads toP. Güttinger’s expression fory>b(t)/2: Zeits. Phys., 73, 169 (1931), eq. (43). The rate of stimulated emission2ybjyjbl/2dt for arbitraryya,yh,\ agrees withS. Rautian andI. Sobel’man:Sov. Phys. JBTP, 14, 328 (1962), eq. (lia). For additional references and another application to lasers, seeJ. A. White:Appl. Phys. Lett., 3, 107 (1963).