Volume 70, Nº 4 (2024)

In this paper, we consider the sum of linear fractional-power operators acting in a Banach space and satisfying weak positivity. We establish the correct solvability of the problem for the corresponding fractional-operator equation and we give the representation of the solution through the inverse operator with an exact estimate of its norm. The results are applied to problems without initial conditions for an equation with singular coefficients. We consider examples of such equations.

In this paper, we present a detailed exposition of Bernstein’s inequality, inequalities of different metrics and different dimensions for trigonometric polynomials in periodic Morrey spaces.

In this paper, we study the feedback control problem for a mathematical model describing the motion of a nonlinear viscous fluid with infinite memory along the trajectories of the velocity field. The existence of an optimal control that gives a minimum to a given bounded and lower semicontinuous quality functional is proved. The proof uses the approximation-topological approach, the theory of regular Lagrangian flows, and the theory of topological degree for multivalued vector fields.

We discuss the connection between the construction of an ordered billiard game introduced earlier by Dragovic and Radnovic and the class of billiard books proposed by Vedyushkina. In this paper, we propose a generalization of the concept of realization of a certain game using a billiard book and prove an analogue of the Dragovic-Radnovic theorem for such a realization. We present recent results by the authors, Tyurina, and Zav’ialov on topological properties of isoenergy manifolds of circular billiard books and topological invariants of specific series of elliptic billiard books.

Using the Balakrishnan-Yosida approach to constructing fractional powers of linear operators in a Banach space by means of strongly continuous semigroups with densely defined generating operators, in this paper, a similar scheme is presented for constructing fractional powers of nondensely defined operators by means of semigroups with a summable singularity. It is found that the newly constructed semigroups also have a singularity at zero, and their sharp estimate is established, related to the order of the singularity of the original semigroup and the fractional power of the constructed operator, in particular, the square root. As an example, the obtained results are applied to semigroups with a singularity given in the paper [3] and in the doctoral dissertation of Yu. T. Silchenko, and a square root is also constructed for a nondensely defined operator.

This work presents a multiscale mathematical model of the spread of respiratory viral infection in a tissue and in an organism, taking into account the influence of innate and adaptive immune responses based on systems of reaction-diffusion equations with nonlocal terms. The defining characteristics of such models, which have physiological significance, are the viral replication number, wave propagation speed, and total viral load. In this work, these characteristics are estimated and their dependence on immune response parameters is investigated.

In this paper, we study the questions of well-posedness of linear inverse problems for equations in Banach spaces with an integro-differential operator of the Riemann-Liouville type and a bounded operator at the unknown function. A criterion of well-posedness is found for a problem with a constant unknown parameter; in the case of a scalar convolution kernel in an integro-differential operator, this criterion is formulated as conditions for the characteristic function of the inverse problem not to vanish on the spectrum of a bounded operator. Sufficient well-posedness conditions are obtained for a linear inverse problem with a variable unknown parameter. Abstract results are used in studying a model inverse problem for a partial differential equation.