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We propose a fast method for high order approximations of the solution of the Cauchy problem for the linear non-stationary Stokes system in $R^3$ in the unknown velocity $\bf u$ and kinematic pressure $P$. The density ${\bf f}({\bf x},t)$ and the divergence vector-free initial value ${\bf g }(\bf x)$ are smooth and rapidly decreasing as $|\bf x|$ tends to infinity. We construct the vector $\bf u$ in the form $\bf u=\bf u_1+\bf u_2$ where $\bf u_1$ solves a system of homogeneous heat equations and $\bf u_2$ solves a system of non-homogeneous heat equations with right-hand side ${\bf f}-\nabla P$. Moreover $P=-{\cal L}( \nabla\cdot \bf f)$ where $\cal L$ denotes the harmonic potential. Fast semi-analytic cubature formulas for computing the harmonic potential and the solution of the heat equation based on the approximation of the data by functions with analitically known potentials are considered. In addition, the gradient $\nabla P$ can be approximated by the gradient of the cubature of $P$, which is a semi-analytic formula too. We derive fast and accurate high order formulas for the approximation of $\bf u_1,\bf u_2$, $P$ and $\nabla P$. The accuracy of the method and the convergence order $2,4,6$ and $8$ are confirmed by numerical experiments.

We propose a fast method for high order approximation of potentials of the Helmholtz type operator Delta+kappa^2 over hyper-rectangles in R^n. By using the basis functions introduced in the theory of approximate approximations, the cubature of a potential is reduced to the quadrature of one-dimensional integrals with separable integrands. Then a separated representation of the density, combined with a suitable quadrature rule, leads to a tensor product representation of the integral operator. Numerical tests show that these formulas are accurate and provide approximations of order 6 up to dimension 100 and kappa^2=100.

We derive new formulas for the high dimensional biharmonic potential acting on Gaussians or Gaussians times special polynomials. These formulas can be used to construct accurate cubature formulas of an arbitrary high order which are fast and effective also in very high dimensions. Numerical tests show that the formulas are accurate and provide the predicted approximation rate (O(h^8)) up to the dimension 10^7.

In this paper we propose fast solution methods for the Cauchy problem for the multidimensional Schr\"odinger equation. Our approach is based on the approximation of the data by the basis functions introduced in the theory of approximate approximations. We obtain high-order approximations also in higher dimensions up to a small saturation error, which is negligible in computations, and we prove error estimates in mixed Lebesgue spaces for the inhomogeneous equation. The proposed method is very efficient in high dimensions if the densities allow separated representations. We illustrate the efficiency of the procedure on different examples, up to approximation order 6 and space dimension 200.

This is in some sense an addendum to the book Relational Mathematics by the first-named author. It originated from work on diverse other topics during which a lot of purely relational results with broad applicability have been produced. These include results on domain construction with novel formulae for existential and inverse image, a relational calculus for binary mappings, and the development of a formally derived relational calculus of Kronecker-, strict fork-, and strict join-operators. The many visualizations in this report make it also a scrap- and picture book for examples., Comment: Tech. Report, Universitaet der Bundeswehr Muenchen, 45 pages

In the present paper we study high-order cubature formulas for the computation of advection-diffusion potentials over boxes. By using the basis functions introduced in the theory of approximate approximations, the cubature of a potential is reduced to the quadrature of one dimensional integrals. For densities with separated approximation, we derive a tensor product representation of the integral operator which admits efficient cubature procedures in very high dimensions. Numerical tests show that these formulas are accurate and provide approximation of order $O(h^6)$ up to dimension $10^8$., Comment: 17 pages

A fast method of an arbitrary high order for approximating volume potentials is proposed, which is effective also in high dimensional cases. Basis functions introduced in the theory of approximate approximations are used. Results of numerical experiments, which show approximation order O(h^8) for the Newton potential in high dimensions, for example, for n= 200 000, are provided. The computation time scales linearly in the space dimension. New one-dimensional integral representations with separable integrands of the potentials of advection-diffusion and heat equations are obtained., Comment: 19 pages

The paper is devoted to the efficient computation of high-order cubature formulas for volume potentials obtained within the framework of approximate approximations. We combine this approach with modern methods of structured tensor product approximations. Instead of performing high-dimensional discrete convolutions the cubature of the potentials can be reduced to a certain number of one-dimensional convolutions leading to a considerable reduction of computing resources. We propose one-dimensional integral representions of high-order cubature formulas for n-dimensional harmonic and Yukawa potentials, which allow low rank tensor product approximations., Comment: 20 pages

In this paper we derive approximate quasi-interpolants when the values of a function $u$ and of some of its derivatives are prescribed at the points of a uniform grid. As a byproduct of these formulas we obtain very simple approximants which provide high order approximations for solutions to elliptic differential equations with constant coefficients.