I have a quick question about the Sturm-Liouville problems.
Does someone has a trick to know wether we have to define $\omega := -\lambda^2$ or $\omega = \lambda^2$, i.e. if $\lambda$ must be positive or negative? I know it can be seen with converting the differential equation into the form $(py')' + qy = r\lambda y$ but it takes time and I need to be fast.
Thank you in advance !