Orthogonal polynomials on a class of planar algebraic curves

We construct bivariate orthogonal polynomials (OPs) on algebraic curves of the form ym = φ(x) in R2 where m = 1, 2 and φ is a polynomial of arbitrary degree d, in terms of univariate semiclassical OPs. We compute connection coefficients that relate the bivariate OPs to a polynomial basis that is itself orthogonal and whose span contains the OPs as a subspace. The connection matrix is shown to be banded and the connection coefficients and Jacobi matrices for OPs of degree 0, . . . , N are computed via the Lanczos algorithm in O(Nd4) operations

Blow up in a periodic semilinear heat equation

Blow up in a one-dimensional semilinear heat equation is studied using a combination of numerical and analytical tools. The focus is on problems periodic in the space variable and starting out from a nearly flat, positive initial condition. Novel results include asymptotic approximations of the solution on different timescales that are, in combination, valid over the entire space and time interval right up to and including the blow-up time. Both the asymptotic analysis and the numerical methods benefit from a well-known reciprocal substitution that transforms the problem into one that does not blow up but remains bounded. This allows for highly accurate computations of blow-up times and the solution profile at the critical time, which are then used to confirm the asymptotics. The approach also makes it possible to continue a solution numerically beyond the singularity. The specific post-blow-up dynamics are believed to be presented here for the first time

Multidomain spectral method for the Gauss hypergeometric function

We present a multidomain spectral approach for Fuchsian ordinary differential equations in the particular case of the hypergeometric equation. Our hybrid approach uses Frobenius’ method and Moebius transformations in the vicinity of each of the singular points of the hypergeometric equation, which leads to a natural decomposition of the real axis into domains. In each domain, solutions to the hypergeometric equation are constructed via the well-conditioned ultraspherical spectral method. The solutions are matched at the domain boundaries to lead to a solution which is analytic on the whole compactified real line R∪∞ , except for the singular points and cuts of the Riemann surface on which the solution is defined. The solution is further extended to the whole Riemann sphere by using the same approach for ellipses enclosing the singularities. The hypergeometric equation is solved on the ellipses with the boundary data from the real axis. This solution is continued as a harmonic function to the interior of the disk by solving the Laplace equation in polar coordinates with an optimal complexity Fourier–ultraspherical spectral method. In cases where logarithms appear in the solution, a hybrid approach involving an analytical treatment of the logarithmic terms is applied. We show for several examples that machine precision can be reached for a wide class of parameters, but also discuss almost degenerate cases where this is not possible

Spurious Resonance in SemiDiscrete Methods for the Korteweg--de Vries Equation

A multiple scales analysis of semidiscrete methods for the Korteweg--de Vries equation is conducted. Methods that approximate the spatial derivatives by finite differences with arbitrary order accuracy and the limiting method, the Fourier pseudospectral method, are considered. The analysis reveals that a resonance effect can occur in the semidiscrete solution but not in the solution of the continuous equation. It is shown for the Fourier pseudospectral discretization that resonance can only be caused by aliased modes. The spurious semidiscrete solutions are investigated in numerical experiments and we suggest methods for avoiding spurious resonance

Orthogonal polynomials on planar cubic curves

Orthogonal polynomials in two variables on cubic curves are considered. For an integral with respect to an appropriate weight function defined on a cubic curve, an explicit basis of orthogonal polynomials is constructed in terms of two families of orthogonal polynomials in one variable. We show that these orthogonal polynomials can be used to approximate functions with cubic and square root singularities, and demonstrate their usage for solving differential equations with singular solutions

Blow up in a periodic semilinear heat equation

Blow up in a one-dimensional semilinear heat equation is studied using a combination of numerical and analytical tools. The focus is on problems periodic in the space variable and starting out from a nearly flat, positive initial condition. Novel results include asymptotic approximations of the solution on different timescales that are, in combination, valid over the entire space and time interval right up to and including the blow-up time. Both the asymptotic analysis and the numerical methods benefit from a well-known reciprocal substitution that transforms the problem into one that does not blow up but remains bounded. This allows for highly accurate computations of blow-up times and the solution profile at the critical time, which are then used to confirm the asymptotics. The approach also makes it possible to continue a solution numerically beyond the singularity. The specific post-blow-up dynamics are believed to be presented here for the first time

Quadratic Pade approximation : numerical aspects and applications

CITATION: Fasondini, M. et al. 2019. Quadratic Pade approximation : numerical aspects and applications. Computer Research and Modeling, 11(6):1017–1031, doi:10.20537/2076-7633-2019-11-6-1017-1031.The original publication is available at http://www.mathnet.ruPad´e approximation is a useful tool for extracting singularity information from a power series. A linear Pad´e approximant is a rational function and can provide estimates of pole and zero locations in the complex plane. A quadratic Pad´e approximant has square root singularities and can, therefore, provide additional information such as estimates of branch point locations. In this paper, we discuss numerical aspects of computing quadratic Pad´e approximants as well as some applications. Two algorithms for computing the coefficients in the approximant are discussed: a direct method involving the solution of a linear system (well-known in the mathematics community) and a recursive method (well-known in the physics community). We compare the accuracy of these two methods when implemented in floating-point arithmetic and discuss their pros and cons. In addition, we extend Luke’s perturbation analysis of linear Pad´e approximation to the quadratic case and identify the problem of spurious branch points in the quadratic approximant, which can cause a significant loss of accuracy. A possible remedy for this problem is suggested by noting that these troublesome points can be identified by the recursive method mentioned above. Another complication with the quadratic approximant arises in choosing the appropriate branch. One possibility, which is to base this choice on the linear approximant, is discussed in connection with an example due to Stahl. It is also known that the quadratic method is capable of providing reasonable approximations on secondary sheets of the Riemann surface, a fact we illustrate here by means of an example. Two concluding applications show the superiority of the quadratic approximant over its linear counterpart: one involving a special function (the Lambert W-function) and the other a nonlinear PDE (the continuation of a solution of the inviscid Burgers equation into the complex plane).Publisher's versio