We study a three-particle Schrόdinger operator H for which none of the two-particle subsystems has negative bound states and at least two of them have zero energy resonances. We prove that under this condition the number N(z) of bound states of H below z < 0 has the asymptotics N(z) ~ 9I 0 |log|z|| as z -> -0, where the coefficient 9I 0 depends only on the ratio of masses of the particles.

We consider a periodic self-adjoint pseudo-differential operator H = (−∆) m + B, m > 0, in R d which satisfies the following conditions: (i) the symbol of B is smooth in x, and (ii) the perturbation B has order less than 2m. Under these assumptions, we prove that the spectrum of H contains a half-line. This, in particular implies the Bethe-Sommerfeld Conjecture for the Schrödinger operator with a periodic magnetic potential in all dimensions.2000 Mathematics Subject Classification. Primary 35P20, 47G30, 47A55; Secondary 81Q10.

Abstract. The article discusses the following frequently arising question on the spectral structure of periodic operators of mathematical physics (e.g., Schrödinger, Maxwell, waveguide operators, etc.). Is it true that one can obtain the correct spectrum by using the values of the quasimomentum running over the boundary of the (reduced) Brillouin zone only, rather than the whole zone? Or, do the edges of the spectrum occur necessarily at the set of "corner" high symmetry points? This is known to be true in 1D, while no apparent reasons exist for this to be happening in higher dimensions. In many practical cases, though, this appears to be correct, which sometimes leads to the claims that this is always true. There seems to be no definite answer in the literature, and one encounters different opinions about this problem in the community.In this paper, starting with simple discrete graph operators, we construct a variety of convincing multiply-periodic examples showing that the spectral edges might occur deeply inside the Brillouin zone. On the other hand, it is also shown that in a "generic" case, the situation of spectral edges appearing at high symmetry points is stable under small perturbations. This explains to some degree why in many (maybe even most) practical cases the statement still holds.

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