Spectral properties of the Kronig-Penney Hamiltonian with a localized impurity

It is shown that there exist bound states of the operator H ±λ=−(d 2/d x 2) +∑ m∈Z δ(⋅−(2m+1)π)±λW, W being an L 1(−∞,+∞) non‐negative function, in every sufficiently far gap of the spectrum of H 0=−d 2/d x 2 +∑ m∈Z δ(⋅−(2m+1)π). Such an operator represents the Schrödinger Hamiltonian of a Kronig–Penney‐type crystal with a localized impurity. The analyticity of the greatest (resp. lowest) eigenvalue of H λ (resp. H −λ) occurring in a spectral gap as a function of the coupling constant λ when W is assumed to have an exponential decay is also proven.