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Let $f \in C^{\infty}([0, 2\pi])$ be a smooth function and consider the following periodic Sturm-Liouville problem:

$$\begin{cases} u''(x) + f(x)u(x) = - \lambda u(x) \\ u(0) = u(2\pi) \\ u'(0) = u'(2\pi) \end{cases}$$

I would like to know if there are mild sufficient conditions on $f$ so that the above problem has a negative simple eigenvalue $\lambda$.

I already know that there are countably many eigenvalues for this problem.

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    $\begingroup$ Subtracting a negative constant from any $f$ that yields a simple eigenvalue is sufficient, but maybe you want to specify better what you are looking for. $\endgroup$
    – Dario
    Commented Nov 10, 2020 at 7:57
  • $\begingroup$ @Dario i edited the post and added a new question. $\endgroup$
    – Eduardo Longa
    Commented Nov 10, 2020 at 15:10

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