This is part of a proof from Sidney Coleman's "Aspects of Symmetry," page 340. We start with the equation $$(\partial_{t}^2 - W(t))\psi = \lambda \psi$$ and define the determinant of the operator as the product of its eigenvalues: $$\det(\partial_{t}^2 - W) = \prod \lambda_n $$. Now, the ratio $$\frac{ \left(\det{\partial_{t}^2 - W^{(1)} - \lambda}\right)}{ \left(\det{\partial_{t}^2 - W^{(2)} - \lambda}\right)}$$ is a meromorphic function of $\lambda$ with a simple zero at each $\lambda_n^{(1)}$ and a simple pole at each $\lambda_n^{(2)}$. The proof then claims that by elementary Fredholm theory, the ratio goes to one as $\lambda \to \infty$. What does Coleman mean here by Fredholm theory? Most of what I have found on Fredholm is about finding solutions to certain integral equations, which doesn't seem relevant here. I am open to either proofs of this statement using the relevant theory, or reference suggestions for where to learn about the appropriate theory. Thanks!