Vol 197, No 3 (2018)

Based on solutions of a system of quasilinear first-order equations of a special kind (rivertons), we construct classes of exact solutions of multidimensional nonlinear Klein–Gordon equations. The obtained solutions are expressed in terms of the derivatives of rivertons with respect to the independent variables. As a result, the solutions are multivalued and have singularities at the branch points. In the general case, the solutions can be complex. We establish a relation between the functional form of the nonlinearity of the Klein–Gordon equations and the functional dependence of the solutions on rivertons and their derivatives. We study the conditions under which the nonlinearity of the Klein–Gordon equation has a specific functional form and present examples. We establish a relation between the geometric structure of rivertons and the initial conditions.

We describe the topology of isoenergetic surfaces for an integrable system on the Lie algebra so(3, 1) and the critical points of the Hamiltonian for different parameter values. We construct bifurcation values of the Hamiltonian.

We discuss properties of R-matrix-valued Lax pairs for the elliptic Calogero-Moser model. In particular, we show that the family of Hamiltonians arising from this Lax representation contains only known Hamiltonians and no others. We review the relation of R-matrix-valued Lax pairs to Hitchin systems on bundles with nontrivial characteristic classes over elliptic curves and also to quantum long-range spin chains. We prove a general higher-order identity for solutions of the associative Yang–Baxter equation.

We previously proposed an approach for constructing integrable equations based on the dynamics in associative algebras given by commutator relations. In the framework of this approach, evolution equations determined by commutators of (or similarity transformations with) functions of the same operator are compatible by construction. Linear equations consequently arise, giving a base for constructing nonlinear integrable equations together with the corresponding Lax pairs using a special dressing procedure. We propose an extension of this approach based on introducing higher analogues of the famous Hirota difference equation. We also consider some (1+1)-dimensional discrete integrable equations that arise as reductions of either the Hirota difference equation itself or a higher equation in its hierarchy.

We discuss the possibility of using the intersection points of the common level surface of integrals of motion with an auxiliary curve to construct finite-difference equations corresponding to different discretizations of the original integrable system. As an example, we consider the generalized one-dimensional oscillator with third- and fifth-degree nonlinearity, for which we show that the intersection divisors of the hyperelliptic curve with straight lines, quadrics, and cubics generate families of integrable discrete maps.

We consider entanglement entropy quantities for a three-part system, namely, the tripartite information, total correlation, and so-called secrecy monotone. A holographic approach is used to calculate the time evolution of the entanglement entropy during nonequilibrium heating, which leads to holographic definitions of these quantities. We study time dependence of these three quantities.

With wide applications in secure data transmission and encryption, synchronization of chaotic systems is an interesting concept and has accordingly received special attention among nonlinear systems. Here, we propose an appropriate controller for synchronizing one-parameter families of piecewise nonlinear chaotic maps using a projective synchronization method. First, we present synchronization in coupled chaos discrete-time systems using the master–slave method. Using the principle of the stability of the Lyapunov function, we design a proper controller for achieving projective synchronization of piecewise nonlinear systems. Finally, we demonstrate the applicability of the proposed scheme with simulation results.