1. L. D. Landau and E. M. Lifshits, Course of Theoretical Physics, Vol. 3: Quantum Mechanics: Non-Relativistic Theory, Pergamon Press, London–Paris (1958).
2. M. V. Fedoryuk, “Asymptotics of the discrete spectrum of the operator $$w''(x)-\lambda^2p(x)w(x)$$ [in Russian],” Mat. Sb. (N. S.), 68(110), 81–110 (1965).
3. C. Herring, “Critique of the Heitler–London method of calculating spin couplings at large distances,” Rev. Modern Phys., 34, 631–645 (1962).
4. E. Harrell, “Double wells,” Commun. Math. Phys., 75, 239–261 (1980).
5. The Subnuclear Series;S. Coleman,1979