Volume 199, Nº 2 (2019)

We classify elementary Darboux—Laplace transformations for semidiscrete and discrete second-order hyperbolic operators. We prove that there are two types of elementary Darboux—Laplace transformations in the (semi) discrete case as in the continuous case: Darboux transformations constructed from a particular element in the kernel of the initial hyperbolic operator and classical Laplace transformations that are defined by the operator itself and are independent of the choice of an element in the kernel. We prove that on the level of equivalence classes in the discrete case, any Darboux—Laplace transformation is a composition of elementary transformations.

In the two-dimensional case, we construct nondegenerate Hamiltonians that describe electron motion in an electromagnetic field and have additional integrals of motion quadratic in momentum. We completely classify the quasi-Stäckel Hamiltonians related to these systems in the cases where the leading approximation in momenta of the additional integral depends quadratically on the coordinates. We consider reductions of such systems that are symmetric under rotation about the z axis.

Using the Lenard recurrence relations and the zero-curvature equation, we derive the modified Belov—Chaltikian lattice hierarchy associated with a discrete 3×3 matrix spectral problem. Using the characteristic polynomial of the Lax matrix for the hierarchy, we introduce a tri gonal curve K_m−2 of arithmetic genus m−2. We study the asymptotic properties of the Baker—Akhiezer function and the algebraic function carrying the data of the divisor near \(P_{\infty_{1}}\), \(P_{\infty_{2}}\), \(P_{\infty_{3}}\), and P₀ on K_m−2. Based on the theory of trigonal curves, we obtain the explicit theta-function representations of the algebraic function, the Baker—Akhiezer function, and, in particular, solutions of the entire modified Belov—Chaltikian lattice hierarchy.

We use the Zakharov—Shabat dressing method to solve the multicomponent short-pulse equation. We obtain dressed solutions of this equation using a Riemann—Hilbert problem. The dressed solutions of the Lax pair and the multicomponent short-pulse equation are expressed in terms of Hermitian projectors. We show that the dressed solutions are related to quasideterminant solutions, and we write K-soliton solutions in terms of quasideterminants. In explicit form, we obtain one- and two-soliton solutions.

We study the Potts model with a zero external field on the Cayley tree. For the antiferromagnetic Potts model with q states on a second-order Cayley tree and for the ferromagnetic Potts model with q states on a kth-order Cayley tree, we show that all periodic Gibbs measures are translation-invariant for all parameter values.

We consider the Klein—Gordon equation with a localized initial condition and describe the transition of the solution from localized to rapidly oscillating when the equation parameter changes.