Inverse problem for a semilinear functional-differential wave equation

The problem of determining source term in a semilinear wave equation is considered. The source term is represented as the product ƒ(u(x, t))p(x), where ƒ(s) is a given function, u(x, t) is a solution to Cauchy problem for wave equation, p(x) is an unknown function. To determine p(x) the additional information on the solution of the Cauchy problem u(α(t), t) = g(t), u(β(t), t) = h(t) is used. Theorems of existence and uniqueness of solution to an inverse problem in the class of continuous functions p(x) and in the class of functions p(x) = po + xq(x), where q(x) is continuous, are proved.

The problem of determining source term in a semilinear wave equation is considered. The source term is represented as the product ƒ(u(x, t))p(x), where ƒ(s) is a given function, u(x, t) is a solution to Cauchy problem for wave equation, p(x) is an unknown function. To determine p(x) the additional information on the solution of the Cauchy problem u(α(t), t) = g(t), u(β(t), t) = h(t) is used. Theorems of existence and uniqueness of solution to an inverse problem in the class of continuous functions p(x) and in the class of functions p(x) = po + xq(x), where q(x) is continuous, are proved.

Subject of research: problems of numerical determination of point sources in inverse heat and mass transfer problems. Purpose of research: description of theoretical results (theorem of existence and uniqueness of solutions of the inverse problem), creation of an algorithm for solving the problem of numerical determination of point sources, study of its properties, numerical implementation of the algorithm and its testing and verification of stability. Objects of research: problems of numerical determination of point sources (right-hand side of a special type) in inverse problems of heat and mass transfer. The sources are specified as a sum of Dirac delta functions with coefficients that depend on time and characterize the power of the corresponding source. They are unknown and are to be determined together with a solution of the equation. The values of the solution at a certain set of points lying inside the domain serve as the overdetermination data. Methods of research: the algorithm is based on the finite element method for spatial variables and the finite difference method for time. The unknown right-hand side is determined at each time layer using the overdetermination condition. Main results of research: description of the algorithm for solving the problem, its properties, results of numerical experiments. In particular, the conditions are described when the algebraic system to which the problem is reduced has a unique solution. The data obtained as a result of the calculations are compared with test examples. The calculations are also carried out with the addition of random noise of various levels to the measurement data. The results show that the solution is stable under random perturbations of the data.

The inverse problem which consists in recovery of a nonlinear source in the equation of hyperbolic type is considered. The solution of the inverse problem is based on the additional boundary condition. The theorem of existence and uniqueness of solution of the inverse problem is proved in small. The convergence of the iterative method for solving the inverse problem is established.

The inverse problems of finding the free term and the coefficient of u(x,t) in a parabolic equation is considered. The Fredholm property of the linear inverse problem of finding the right-hand side of a special form is proved, along with global existence, uniqueness, and stability theorems for its solutions. A uniqueness theorem is established for the nonlinear inverse problem of determining the coefficient under restrictions in the form of inequalities containing no smallness conditions. The proof is carried out by a method of a priori estimates, with the use of the maximum principle for parabolic and elliptic equations. A connection between the uniqueness of the solution of the inverse problem and the completeness of a certain system of functions is established.

Solution of the fourth and fifth boundary value problems for a one-dimensional second-order parabolic equation in a curvilinear region

The work is devoted to the study of the solvability of the inverse boundary value problem with an unknown time depended coefficient for a sixth-order Boussinesq equation with an additional integral condition of the first kind is investigated. First, the given problem is reduced to an equivalent problem in a certain sense. Then, using the Fourier method the equivalent problem is reduced to solving the system of integral equations. The existence and uniqueness of a solution to the system of integral equation is proved by the contraction mapping principle. This solution is also the unique solution to the equivalent problem. Finally, by equivalence, the theorem of existence and uniqueness of a classical solution to the given problem is proved. The sixth order Boussinesq-Type problem is discretized using the Quintic B-spline (QB-spline) collocation technique and reshaped as non-linear least-squares optimization of the Tikhonov regularization function. This is numerically solved by means of the MATLAB subroutine lsqnonlin tool. Both perturbed data and analytical solution are inverted. Numerical outcomes are reported and discussed. In addition, the von Neumann stability analysis for the proposed numerical approach has also been discussed.

In this paper we consider the semi-linear wave equation: utt − Δu = ut|ut|p−1 in where and p < 3 if N = 1, p ≠ 3 if N = 2. We give an energetic criteria and optimal lower bound for blow-up solutions of this equation.

We consider the semilinear curl-curl wave equation $$s\left( x \right)\partial _t^2U + \nabla \times \nabla \times + q\left( x \right)U \pm V\left( x \right){\left| U \right|^{p - {\text{ 1}}}}U = 0for\left( {x,t} \right) \in {\mathbb{R}^3} \times \mathbb{R}.$$ For any p < 1 we prove the existence of time-periodic spatially localized real-valued solutions (breathers) both for the + and the - case under slightly different hypotheses. Our solutions are classical solutions that are radially symmetric in space and decay exponentially to 0 as |x| → ∞. Our method is based on the fact that gradient fields of radially symmetric functions are annihilated by the curl-curl operator. Consequently, the semilinear wave equation is reduced to an ODE with r = |x| as a parameter. This ODE can be efficiently analyzed in phase space. As a side effect of our analysis, we obtain not only one but a full continuum of phase-shifted breathers U(x; t + a(x)), where U is a particular breather and a: ℝ3 → ℝ an arbitrary radially symmetric C2-function.

An inverse boundary value problem for a third-order pseudo-hyperbolic equation with integral condition of the first kind is investigated.To study the solvability of the inverse problem, we first reduce the considered problem to an auxiliary problem and prove its equivalence (in a certain sense) to the original problem.Then using the Banach fixed point principle, the existence and uniqueness of a solution to this problem is shown.Further, on the basis of the equivalency of these problems the existence and uniqueness theorem for the classical solution of the inverse coefficient problem is proved for the smaller value of time.

The inverse problem of finding the coefficients q(s) and p(s) in the equation u tt = a 2 u xx + q(u)u t − p(u)u x is investigated. As overdetermination required in the inverse setting, two additional conditions are set: a boundary condition and a condition with a fixed value of the timelike variable. An iteration method for solving the inverse problem is proposed based on an equivalent system of integral equations of the second kind. A uniqueness theorem and an existence theorem in a small domain are proved for the inverse problem to substantiate the convergence of the algorithm.

Existence and uniqueness theorems for periodic Markov process and applications to stochastic functional differential equations

For the beam vibration equation, inverse problems are studied to find the right side, i.e. vibration source. Solutions of the problems by methods of spectral analysis and Volterra integral equations are constructed explicitly as sums of series, and the corresponding uniqueness and existence theorems are proved. When substantiating the existence of a solution to the inverse problem by determining the factor of the right-hand side, which depends on the spatial coordinate, the problem of small denominators arises. In this regard, estimates of the denominators are established that guarantee their separation from zero, with an indication of the corresponding asymptotics. On the basis of these estimates, the convergence of the series in the class of regular solutions of the beam oscillation equation is substantiated.

In Chap. 17, n-th-order functional differential equations are considered. In order to create a concept of nonoscillation for functional differential equations which will be an analogue of nonoscillation theory for ordinary differential equations, we define a nonoscillation interval for the n-th-order homogeneous functional differential equation as interval, where each nontrivial solution can have at most n−1 zeros, counting every zero with its multiplicity. The study of many classical questions of the qualitative theory of n-th-order equations such as existence and uniqueness of solutions of the interpolation boundary value problems, positivity or corresponding regular behavior of their Green’s functions, maximum principles and stability, was considered in the general framework based on the notion of nonoscillation intervals of corresponding linear functional differential equations. It is demonstrated that if the Wronskian of the fundamental system is not equal to zero at the initial point of the interval, then the nonoscillation interval exists for functional differential equations; this is true for a particular case of the n-th-order functional differential equations with Volterra operators (delay equations). For second-order functional differential equations, nonvanishing Wronskian is equivalent to the Sturm theorem on separation of zeros (between two adjacent zeros of any nontrivial solution there is a zero of every other solution). The analogue of the classical Mammana theorem, establishing necessary and sufficient nonoscillation conditions through nonvanishing normal chains of the Wronskians, is proven for n-th-order functional differential equations.

An existence theorem is proved for closed convex surfaces whose principal radii of curvature regarded as functions of the unit normal vectorn satisfy the equation $$R_1 R_2 + \Phi (R_1 + R_2 ,R_1 ,R_2 ,n) + c \cdot n = \varphi (n),$$ wheree is an unknown constant vector being sought along with the surface. This equation has been studied by Miranda, who proved a variety of existence and uniqueness theorems about its solutions. The results obtained lead to a certain simplification in the hypotheses imposed on the functions Φ andϕ in these theorems.