A. Voros, “The return of the quartic oscillator. The complex WKB method,” Ann. Inst. H. Poincar ́e A 39, 211 (1983). [3] E. Witten, “Instantons, The Quark Model, And The 1/N Expansion,” Nucl. Phys. B 149, 285 (1979). [4] J. Zinn–Justin, Quantum field theory and critical phenomena, Oxford University Press, Oxford, 2002. [5] [1] S. Coleman. The uses of instantons. volume the whys of subnuclear physics, Editor A. Zichichi, 1979.