Russian Mathematics

We consider a nonlocal problem for a system of loaded differential equations of the Sobolev type with a multipoint constraint. By introducing additional unknown functions, we reduce the problem under consideration to an equivalent problem consisting of a nonlocal multipoint problem for a system of loaded hyperbolic equations of the second order with functional parameters and integral correlations. We propose algorithms for solving the equivalent problem. Moreover, we establish conditions for the well-posedness of the nonlocal multipoint problem for the system of loaded hyperbolic equations of the second order and conditions for the existence of a unique classical solution to the nonlocal problem for the system of differential equations of the Sobolev type with a multipoint constraint.

We use constructions from the general theory of boundary value problems to build a theory of generalized boundary value problems for the generalized Poisson equation. Namely, generalized solutions of various boundary value problems are introduced and studied for the matrix generalization of the Poisson equation, a description of the set of all such well-posed problems is given. The obtained results are used for advancements in the original general theory of boundary value problems.

We study a steady 3D flow of the ideal gas. In the flow between the bow shock wave and the nose part of the body streamlined by the uniform supersonic flow, we consider isoentropic stream surfaces that originate at closed lines on the shock and envelope its leading point. We show that each vortex line is closed and once envelopes the isoentropic stream surface. We obtain the integral invariant of isoentropic stream surfaces, namely, the circulation of the scaled (in a certain way) vorticity vector over (closed) vortex lines. This result is a 3D generalization of the streamline invariant obtained by L. Crocco for axisymmetric flows.

We propose an effective construction of Padé approximation for the hypergeometric function of special type and its derivative with respect to parameter and this parameter is included both in numerator and denominator of the general term of the corresponding series. The aforementioned construction has been realized by means of a modification of the method that was applied earlier in simpler situations. Thus obtained approximation is made use of afterwards to establish the lower estimates of the numerical linear forms in the values of the functions under consideration. One could obtain similar estimates by the known in the theory of transcendental numbers Siegel’s method, however the estimates obtained by this method would be of less exactness.

We study the Cauchy problem for a linear differential-algebraic system of equations with concentrated delay. Our research continues investigation of solvability of linear Noether boundary value problems for systems of functional-differential equations given in the monographs by A.D. Myshkis, N.V. Azbelev, V.P. Maksimov, L.F. Rakhmatullina, A.M. Samoilenko, and A.A. Boichuk; meanwhile, we use essentially the tool of Moore-Penrose inverse matrices. For a linear differential-algebraic system with concentrated delay, we find sufficient conditions for its solvability and give a construction of generalized Green’s operator for Cauchy’s problem. We also give some examples which illustrate in detail the solvability conditions and the suggested construction.

Existence and uniqueness of a strong solution of the initial-boundary value problem for a system of the motion equations of a nonlinear viscoelastic fluid being a fractional analogue of the Voigt model are established in the plane case.