1. Maslov, V.P.: Asymptotic Methods and Perturbation Theory. MGU (1965)
2. Maslov, V.P., Fedoriuk, M.V.: Quasiclassical Approximation for the Equations of Quantum Mechanics. Nauka (1976)
3. Davies, E.B.: Pseudospectra of differential operators. Oper. Theory 4(3), 243–262 (2000)
4. Evgrafov, M.A., Fedoriuk, M.V.: Asymptotic behavior of solutions of the equation $$w^{\prime \prime }-p(z,\lambda )w=0$$ as $$\lambda \rightarrow \infty $$ in the complex $$z$$-plane. Uspekhi Mat Nauk 21(2), 3–50 (1966)
5. Fedoryuk, M.V.: Asymptotic Analysis: Linear Ordinary Differential Equations. Springer, Berlin (1993)