“…For the first method, we consider the time-independent degenerate perturbation theory applied to N-dimensional infinite cubical well that is perturbed with some perturbation, H 0 . The reason behind the selection of degenerate perturbation theory lies on its wide recent applications in various settings: In studying degeneracy breaking due to short-ranged impurities in atomically doped carbon nanotubes (Bondarev and Lambin, 2005;McCann and Fal'ko, 2005), in investigating the effects of magnetic anisotropy in magnetic molecules (Kostantinidis and Coffey, 2002;Meng and Wessel, 2008;Bialynicki and Sowinski,2007;Bergman et al, 2007), in discussing quantum systems coupled to oscillators (Muthukumar and Mitra, 2002;Hagelstein et al, 2008), in dealing with manybody interacting systems (Stauber et al, 2000;Morawetz, 2002), in studying degenerate atomic systems (Vilain et al, 2001), and in calculating relativistic corrections for low-lying excited states when a hydrogen atom is placed in a strong magnetic field (Poszwa and Rutkowski, 2004). For the second method we choose the sudden approximation and apply it to b-decay of tritium atom.…”