When we extend the previous calculation to two and three dimensions, the
principal features of the solution remain the same, but an important new feature
is introduced. In this section we show how this occurs; this new feature, known
as degeneracy, will turn out to be very important in our study of atomic physics.
To begin with, we need a Schr¨odinger equation that is valid in more than one
dimension; our previous version, Eq. 5.5, included only one spatial dimension. If
the potential energy is a function of x and y, we expect that ψ also depends on
both x and y, and the derivatives with respect to x must be replaced by derivatives
with respect to x and y. In two dimensions, we then have∗