Suppose we are working on a bounded interval $[a,b]$ with Sturm-Liouville operator $L$ given by
$$
 Lf = \frac{1}{w(x)}\left[-\frac{d}{dx}\left(p(x)\frac{df}{dx}\right)+q(x)f\right].
$$

How can I prove that the eigenvalues of the operator can be ordered as an increasing sequence such that : $\lambda\_0 < \lambda\_1 < \lambda\_2... < \lambda\_n < ... \to + \infty$