Gives conditions under which $L$ is an invertible operator.
$L:u \rightarrow -u''+p(x)u'+q(x)u$
$u \in dom(L)= \{u\in C^{2}[a,b], u'(a)-\theta_{a}u(a)=0, u'(b)+\theta_{b}u(b)=0 \} $
$\theta_{a}, \theta_{b} \ge 0$
I thought to check if the operator is positive with these condition, but I don't know if it's a good way. I'm looking for some tips. Thanks in advance.