I'm looking for literature about second order differential equations of the form
$$ -u''(x) + \frac{c}{\sin^2(x)}u = 0,$$
where $u : [0, \pi] \to \mathbb R$ is at least $C^2([0,\pi])$ and $c > 0$ is a constant.
Actually, I'm mostly interested in the subsolutions of this equations. Do you know where to find something about this or how to approach such a problem?