Vol 54, No 2 (2018)

Applying bifurcation theory methods to nonlinear boundary value problems for ordinary differential equations (ODEs) of the fourth and higher orders encounters technical difficulties related to studying the spectrum of direct and adjoint linearized problems and constructing Green functions (i.e., proving the spectral problems to be Fredholm and determining the manifolds of bifurcation points). In order to overcome these difficulties, methods for separating the roots of relevant characteristic equations have been proposed, with the subsequent representation of the bifurcation manifolds in terms of these roots; this enables investigation of nonlinear problems in their rigorous statement. Such an approach is considered using the example of a two-point boundary value problem for a fourth-order ODE in which a statically bent pipeline section is described as a flexible elastic hollow rod, with a liquid flowing inside it, that is compressed or stretched by external boundary conditions with a free sliding left end and fixedly mounted right ends.

The eigenvalue problem is studied for a quasilinear second-order ordinary differential equation on a closed interval with Dirichlet’s boundary conditions (the corresponding linear problem has an infinite number of negative and no positive eigenvalues). An additional (local) condition imposed at one of the endpoints of the closed interval is used to determine discrete eigenvalues. The existence of an infinite number of (isolated) positive and negative eigenvalues is proved; their asymptotics is specified; a condition for the eigenfunctions to be periodic is established; the period is calculated; and an explicit formula for eigenfunction zeroes is provided. Several comparison theorems are obtained. It is shown that the nonlinear problem cannot be studied comprehensively with perturbation theory methods.

A solution of the Dirichlet problem for a fractional-order ordinary differential equation has been found. Green’s function has been constructed for the problem concerned. The problem solution has been written in terms of Green’s function. A theorem on the existence and uniqueness of a solution of the posed problem has been proved, and a condition for its unique solvability has been derived. It is shown that the condition of solvability may only be violated a finite number of times.

A nonlocal problem with integral conditions is considered for the system of partial differential equations of the hyperbolic type in a rectangular domain. Sufficient conditions are established for the existence of the unique classical solution of the studied problem in terms of initial data. An algorithm is proposed for finding a sequence of approximate solutions convergent to the exact solution of the problem. Special cases of the problem at hand are considered as an application of the results obtained.

The first boundary value problem is studied for a linear hyperbolic equation in a rectangle with a power degeneracy on one of its sides for different degrees of degeneracy. A uniqueness criterion is established. The solution is constructed as the sum of a Fourier series. The problem of small denominators arises when justifying the convergence of this series. In this connection, some estimates for the separation of the small denominators from zero are established with the corresponding asymptotics specified. It is these asymptotics that have made it possible to substantiate the existence of a regular problem solution.

Boundary value problems are considered for degenerating and nondegenerating differential equations of the Sobolev type with a nonlocal source as well as finite-difference methods for solving these problems. A priori estimates are derived for solving the problems posed in differential and difference interpretations. These a priori estimates entail the uniqueness and stability of the solution with respect to the initial data and the right-hand side on a layer as well as the convergence of the solution of each difference problem to that of the counterpart differential problem.

A mixed problem is considered for a differential equation with an involution and matrix potential. The method of similar operators is used to transform the differential operator defined by this equation into the orthogonal direct sum of finite-rank operators. A theorem is established, based on which an operator group is constructed to describe mild solutions of the problem at hand.