Expansion of the H2+-ground state energy in inverse powers of the distance between the two protons

The ground state energy of the ionized hydrogen molecule H2+ has been expanded in inverse powers of the distance between the two protons E = Σ asub(n)/Rsup(n). It is explained here that for n very large the coefficients asub(n) behave as asub(n) approximately equal to -2/e2 factorial (n+1)/2sup(n+1)