[1] T. Kato. Perturbation Theory for Linear Operators(1966).
[3] M. Berry. Physics of nonhermitian degeneracies. Czech. J. Phys., 54, 1039-1047(2004).
[39] Y.-H. Lai, Y.-K. Lu, M.-G. Suh, K. Vahala. Enhanced sensitivity operation of an optical gyroscope near an exceptional point(2019).
[54] H. Eleuch, I. Rotter. Resonances in open quantum systems. Phys. Rev. A, 95, 022117(2017).