A second-order differential equation

$\frac{\text{d}}{\text{d}x}\left[p(x)\frac{\text{d}y}{\text{d}x}\right]+\left[\lambda \omega (x)-q(x)\right]y=0$

where ω‎(x) is called the ‘weighting function’. The values of λ‎ for which there is a solution are the eigenvalues and the corresponding solutions are the eigenfunctions.