CONVOLUTION EQUATIONS ON THE ABELIAN GROUP $\cA(-1,1)$

The interval $j=[-1,1]$ turns into an Abelian group $\cA(\cJ)$ under the group operation $x+_\cJ y:=(x+y)(1+xy)^{-1},\qquad x,y\in\cJ$. This enables definition of the invariant measure $d_\cJ x=(1-x^2)^{-1}dx$ and the Fourier transform $\cF_\cJ$ on the interval $\cJ$ and, as a consequence, we can consider Fourier convolution operators $W^0_{\cJ,\cA}:=\cF_\cJ^{-1}\cA\cF_\cJ$ on $\cJ$. This class of convolutions includes celebrated Prandtl, Tricomi and Lavrentjev-Bitsadze equations and, also, differential equations of arbitrary order with the natural weighted derivative $\fD_\cJ u(x)=-(1-x^2)u’(x)$, $t\in\cJ$. Equations are solved in the scale of Bessel potential $\bH^s_p(\cJ,d_\cJ x)$, $1\leqslant p\leqslant\infty$, and H\”older-Zygmound $\bZ^\nu(\cJ,(1-x^2)^\mu)$, $0<\mu,\nu<\infty$ spaces, adapted to the group $\cA(\cJ)$. Boundedness of convolution operators (the problem of multipliers) is discussed. The symbol $\cA(\xi)$, $\xi\in\bR$, of a convolution equation $W^0_{\cJ,\cA}u=f$ defines solvability: the equation is uniquely solvable if and only if the symbol $\cA$ is elliptic. The solution is written explicitely with the help of the inverse symbol. We touch shortly the multidimensional analogue-the Abelian group $\cA(\cJ^n)$.

Solvability Criterion for Convolution Equations on a Half-Strip

We obtain the criterion of solvability of homogeneous convolution equation in a half-strip. Proof is based on a new decomposition property of the weighted Hardy space. This result has relations to the spectral analysis-synthesis problem, cyclicity problem, information theory. All data generated or analysed during this study are included in this published article.

SIMPLIFIED MODEL OF LINEAR INERTIAL MEASUREMENT SYSTEMS

Problem. To increase the metrological reliability of measuring systems at technical objects, the number of sensors measuring the same process parameter is increased to several units and a model of a multi-channel measuring system is synthesized. This synthesis is usually based on the use of Markov's theory of linear filtering, but the presence of a connection between the input and output signals of the linear inertial system through the convolution integral significantly complicates the process of obtaining the optimal device. Goal. The aim of the article is to develop a method for approximating the integral equation of convolution, which describes a linear inertial system, and to estimate the limits of its application on the example of linear inertial sensors. Methodology. Instead of the output signal in the form of a convolution equation, the output signal, defined as the product of the input signal to an unknown time function is used. This function is represented by the Karhunen-Loev series. The distance in the functional space with a quadratic metric between these output signals is minimized by means of a genetic algorithm and the coefficients of the series and, therefore, the unknown function itself, are determined. Results. In the simulation, the relative difference between the output signals, which were calculated from exact and simplified expressions, was determined. Realizations of stationary signals were used as input signals, and the pulse characteristics of the linear inertial system varied over a wide range. The errors of approximation of the integral convolution equation by a simple model do not exceed a few percent. Origina-lity. The approximation of the convolution equation by a simplified model of the system is original and, although it cannot be applied in a wide range of conditions, it is acceptable for a separate class of stationary signals without restrictions. The accuracy of the approximation of the convolution equation is greatest if the width of the spectrum of the input signals is less than the bandwidth of the measuring system. Practical value. The obtained connection between input and output signals based on a simplified model allows to synthesize multi-channel measuring systems using advanced Markov filtering methods for a separate class of stationary input signals. To expand the application of the method in a wide range of conditions, a set of simplified models that are created in advance can be used.

Smoothness of solutions of a convolution equation of restricted type on the sphere

Let $\mathbb {S}^{d-1}$ denote the unit sphere in Euclidean space $\mathbb {R}^d$ , $d\geq 2$ , equipped with surface measure $\sigma _{d-1}$ . An instance of our main result concerns the regularity of solutions of the convolution equation $$\begin{align*}a\cdot(f\sigma_{d-1})^{\ast {(q-1)}}\big\vert_{\mathbb{S}^{d-1}}=f,\text{ a.e. on }\mathbb{S}^{d-1}, \end{align*}$$ where $a\in C^\infty (\mathbb {S}^{d-1})$ , $q\geq 2(d+1)/(d-1)$ is an integer, and the only a priori assumption is $f\in L^2(\mathbb {S}^{d-1})$ . We prove that any such solution belongs to the class $C^\infty (\mathbb {S}^{d-1})$ . In particular, we show that all critical points associated with the sharp form of the corresponding adjoint Fourier restriction inequality on $\mathbb {S}^{d-1}$ are $C^\infty $ -smooth. This extends previous work of Christ and Shao [4] to arbitrary dimensions and general even exponents and plays a key role in the companion paper [24].

Calculation of Safety Factors of the Eurocodes

This study concerns the safety factor and the reliability calculation for structural codes. The Eurocodes are used as a reference. Safety factor calculation is a demanding task which necessitates using an appropriate root-solving algorithm with a sufficient numerical accuracy. This article introduces a simple algorithm to calculate the safety factors directly, as previously there has been no means to control the accuracy. Presently, the safety factors are defined indirectly through the reliability index. The basic safety factor calculation is presented here in six different equations with the same outcome but differences regarding the numerical calculation, which provides a method to check the accuracy and select a proper equation for the root solver. The safety factor calculation for the permanent and the variable load in the Eurocodes is based on the independent, i.e., random, load combination and single load pairs. The current approach of safety factor calculation applied in the Eurocodes is disclosed here. Simple analytical equations based on the convolution equation are presented. Those can be used instead of the computer programs applied currently.

Existence and monotonicity of nonlocal boundary value problems: the one-dimensional case

We consider nonlocal equations of the general form \begin{equation} \left(a*u''\right)(\cdot)+\lambda f\big(\cdot,u(\cdot)\big)=0.\nonumber \end{equation} By developing a Green's function representation for the solution of the boundary value problem we study existence, uniqueness, and qualitative properties (e.g., positivity or monotonicity) of solutions to these problems. We apply our methods to fractional order differential equations. We also demonstrate an application of our methodology both to convolution equations with nonlocal boundary conditions as well as those with a nonlocal term in the convolution equation itself.