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∗