Spectral estimates for periodic fourth order operators

We consider the operator H = d 4 dt4 + d dtp d dt + q with 1-periodic coefficients on the real line. The spectrum of H is absolutely continuous and consists of intervals separated by gaps. We describe the spectrum of this operator in terms of the Lyapunov function, which is analytic on a two-sheeted Riemann surface. On each sheet the Lyapunov function has the standard properties of the Lyapunov function for the scalar case. We describe the spectrum of H in terms of periodic, antiperiodic eigenvalues, and so-called resonances. We prove that 1) the spectrum of H at high energy has multiplicity two, 2) the asymptotics of the periodic, antiperiodic eigenvalues and of the resonances are determined at high energy, 3) for some specific p the spectrum of H has an infinite number of gaps, 4) the spectrum of H has small spectral band (near the beginner of the spectrum) with multiplicity 4 and its asymptotics are determined as p → 0, q = 0.