Some families of mathematical models of biological populations are considered. Invariant ratios between the parameters which characterize this or that population are revealed. Dynamic properties of models are investigated on the assumption that one or several populations are strongly prolific, which means that the corresponding malthusian coefficients are rather great. On the basis of a special asymptotic method developed by the author a problem of behavior of initial system solutions can be reduced to a significantly simpler problem of dynamics of the finite-dimensional mappings received. In particular, it is shown that irregular relaxation vibrations are typical for the solutions of these mappings and, as a result, for the solution of the initial equation systems. It is interesting to note that these viabrations are of big amplitudes.

In this paper, we describe the features of oscillations in adiabatic oscillators when the delay is introduced into the equation. We give a short description of the method of asymptotic integration of one class of linear delay differential systems in the neighborhood of infinity. This method is based on the idea of transforming the initial system in order to reduce it to the system that is close in some sense to the system of ordinary differential equations. When applying this method, we need to extend the phase space of the initial system. The averaging changes of variables are also used to simplify the procedure of constructing the asymptotic formulas. Finally, we apply the functional differential analog of the Levinson theorem. We use this method to get the asymptotic formulas for adiabatic oscillators with delay under a monotonely and also oscillatory tending to zero perturbations. In conclusion, we study the transformation of the parametric resonance zone of one adiabatic oscillator when the delay is varied.

The dynamics of class B lasers with the incoherent optical feedback formed by quickly vibrating external mirrors is viewed. The problem of the stability of equilibrium in a model system with rapidly oscillating coefficients is studied. The averaged system with the distributed delay is received. It is determined that in the presence of fast delay oscillation the limit of instability of a balance state moves towards significantly greater values of the feedback coefficient. The dependence of the shift with increasing the amplitude modulation has a band structure, so the rapid oscillations of delay can stabilize or destabilize the equilibrium. Normal forms which show changes of the sign of Lyapunov quantityalong border are constructed. They describe characteristics of periodic and quasiperiodic modes close to the balance state.

We continue the study of extending the concept of invariance sets relative to control systems and differential inclusions. This expansion consists in studying statistically invariant sets and statistical characteristics of the attainability set of control systems. In this work, we obtain conditions for the statistical invariance and investigate the properties of the statistical characteristics of control systems with periodic coefficients. It is shown that the property of statistical invariancy is closely connected with the property of admissibility of periodic processes for linear control systems. The admissibility means that for any periodic control from the fixed set there exists a unique periodic solution which is in the given set of the phase space. The results of the work can be applied while finding the statistical characteristics arising in various models of biology, chemistry, economy.

In this article, it is considered some questions of approximation solving of an optimal control problem for nonlinear partial pseudohyperbolic differential equations of the fifth order with initial-boundary value conditions and general view of the optimality criterion. Using the method of separation of variables in the form of a Fourier series reduces the generalized solution of the initial-boundary value problem to a countable system of nonlinear integral equations. By the aid of the methods of successive approximations and integral inequalities it is studied the one-value solvability of a finite system of nonlinear integral equations for the fixed values of the control, which are bounded by the given positive constant. It is estimated the permissible error with respect to a state of a ”shorter” generalized solution of the initial-boundary value problem. Further, it is proved that the control sequence is a minimizing sequence for the considered problem.

In this paper, some exact inequalities between the best approximations of periodic differentiable functions with trigonometric polynomials and generalized moduli of the continuity Ωm of m-th order in L₂[0, 2π] space are found. Similar averaged characteristics of function smoothness in studying the important problems in the constructive theory of functions were considered by K.V. Runovskiy, E.A. Strogenko, V.G. Krotov, P. Osvald and many others. For some classes of functions defined by indicated moduli of continuity where the r-th derivatives are bounded by functions which satisfy certain constraints were obtained the exact values of Bernstein, Gelfand, Kolmogorov, linear and projection n-widths. Here is given an example of a majorant for which all the stated claims are fulfilled.

In this paper, it is considered the extremal problem of finding the exact constants in inequalities of Jackson – Stechkin type between the best approximations of periodic differentiable functions f ∈ L (r) 2 [0, 2π] by trigonometric polynomials, and the average values with a positive weight ϕ moduli of continuity of mth order ωm(f (r) , t), belonging to the space Lp, 0 < p ≤ 2. In particular, the problem of minimizing the constants in these inequalities over all subspaces of dimension n, raised by N.P. Korneychuk, is solved. For some classes of functions defined by the specified moduli of continuity, the exact values of n-widths of class

 L (r) 2 (m, p, h; ϕ) :=    f ∈ L (r) 2 :   Z h 0 ω p m(f (r) ;t)2 ϕ(t)dt   1/p  Z h 0 ϕ(t)dt   −1/p ≤ 1   

are found in the Hilbert space L2, and the extreme subspace is identified. In this article, the results are shown which are the extension and the generalization of some earlier results obtained in this line of investigation.

Closed locally minimal networks can be viewed as “branching” closed geodesics. We study such networks on the surfaces of convex polyhedra and discuss the problem of describing the set of all convex polyhedra that have such networks. A closed locally minimal network on a convex polyhedron is an embedding of a graph provided that all edges are geodesic arcs and at each vertex exactly three adges meet at angles of 120∘ . In this paper, we do not deal with closed (periodic) geodesics. Among other results, we prove that the natural condition on the curvatures of a polyhedron that is necessary for the polyhedron to have a closed locally minimal network on its surface is not sufficient. We also prove a new stronger necessary condition. We describe all possible combinatorial structures and edge lengths of closed locally minimal networks on convex polyhedra. We prove that almost all convex polyhedra with vertex curvatures divisible by π/3 have closed locally minimal networks.

We study a problem of a number of lattice plane tilings by given area polyominoes. A polyomino is a connected plane geometric figure formed by joining edge to edge a finite number of unit squares. A tiling is a lattice tiling if each tile can be mapped to any other tile by translation which maps the whole tiling to itself. Let T(n) be a number of lattice plane tilings by given area polyominoes such that its translation lattice is a sublattice of Z². It is proved that 2n−3 + 2[ n−3 2 ] ≤ T(n) ≤ C(n + 1)3 (2.7)n+1. In the proof of a lower bound we give an explicit construction of required lattice plane tilings. The proof of an upper bound is based on a criterion of the existence of lattice plane tiling by polyomino and on the theory of self-avoiding walk. Also, it is proved that almost all polyominoes that give lattice plane tilings have sufficiently large perimeters.

We consider a system of three unidirectionally coupled singularly perturbed scalar nonlinear differential-difference equations with two delays that simulate the electrical activity of the ring neural associations. It is assumed that for each equation at critical values of the parameters there is a case of an infinite dimensional degeneration. Further, we constructed a quasi-normal form of this system, provided that the bifurcation parameters are close to the critical values and the coupling coefficient is suitably small. In analyzing this quasi-normal form, we can state on the base of the accordance theorem, that any preassigned finite number of stable periodic motions can co-exist in the original system under the appropriate choice of the parameters in the phase space.