In quantum physics, when finding the solution for a radial equation for a hydrogen atom, you need to keep the function of r finite as r approaches infinity to prevent the solution from becoming unphysical. You can accomplish this by putting constraints on the allowable values of the energy, and causing the solution for the radial equation to go to zero as r goes to infinity.

The problem of having

go to infinity as r goes to infinity lies in the form you assume for f(r), which is

The solution is to say that this power series must terminate at a certain index, which you call N. N is called the radial quantum number. So this equation becomes the following (note that the summation is now to N, not infinity):

For this series to terminate, aN+1, aN+2, aN+3, and so on must all be zero. The recurrence relation for the coefficients ak is

For aN+1 to be zero, the factor multiplying ak–1 must be zero for k = N + 1, which means that

Substituting in k = N + 1 gives you

And dividing by 2 gives you

Making the substitution

where n is called the principal quantum number, gives you

This is the quantization condition that must be met if the series for f(r) is to be finite, which it must be, physically:

Because

the equation

puts constraints on the allowable values of the energy.