Orthogonal structure on a quadratic curve

Orthogonal polynomials on quadratic curves in the plane are studied. These include orthogonal polynomials on ellipses, parabolas, hyperbolas and two lines. For an integral with respect to an appropriate weight function defined on any quadratic curve, an explicit basis of orthogonal polynomials is constructed in terms of two families of orthogonal polynomials in one variable. Convergence of the Fourier orthogonal expansions is also studied in each case. We discuss applications to the Fourier extension problem, interpolation of functions with singularities or near singularities and the solution of Schrödinger’s equation with nondifferentiable or nearly nondifferentiable potentials.

To the orthogonal expansion theory of the solution to the Cauchy problem for second-order ordinary differential equations

Доказана теорема о разрешимости нелинейной системы уравнений относительно приближенных значений коэффициентов Чебышёва старшей производной, входящей в дифференциальное уравнение. Теорема является теоретическим обоснованием ранее предложенного приближенного метода интегрирования канонических систем обыкновенных дифференциальных уравнений второго порядка на основе ортогональных разложений с использованием многочленов Чебышёва первого рода. A solvability theorem is proved for a nonlinear system of equations with respect to the approximate Chebyshev coefficients of the highest derivative in an ordinary differential equation. This theorem is a theoretical substantiation for the previously proposed approximate method of solving canonical systems of second-order ordinary differential equations using orthogonal expansions on the basis of Chebyshev polynomials of the first kind.