Let L 2 [0, ω] be the Hilbert space of complex measurable (classes of) functions whose modulus is square-summable with inner product of the formWe consider the Hill differential operatorIt is assumed that the potential v belongs to the Hilbert space L 2 [0, ω] and its Fourier series expansionis used everywhere below. The operator L θ , θ ∈ [0, 1], can be expressed asThe operator V is the operator of multiplication by the potential v with domainThe operator L 0 θ is a self-adjoint nonnegative operator with compact resolvent.The spectrum σ(L 0 θ ) of the operator L 0 θ is described as follows:(a) for the case θ ∈ {0, 1}, the eigenvalues are of the form λ n = λ n,θ = π(2n + θ) ω 2 ,