Rayleigh - Schrödinger perturbation series
for cubic anharmonic oscillator x²/2 + g^1/2 x³

H = H₀ + g^1/2 x³	Hamiltonian of the problem
E(g) = E₀ + E₁ g + ... + E_N g^N	Perturbation expansion of energy
H₀ = p²/2 + x²/2	Hamiltonian of harmonic oscillator
E₀ = n + 1/2	Unperturbed harmonic-oscillator energy
g	Small perturbation parameter
n	Harmonic oscillator quantum number
N	Order of perturbation theory

Rayleigh - Schrödinger perturbation series for cubic anharmonic oscillator x2/2 + g1/2 x3