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On the number of stationary patterns in reaction-diffusion systems
Rybář, Vojtěch ; Vejchodský, Tomáš
We study systems of two nonlinear reaction-diffusion partial differential equations undergoing diffusion driven instability. Such systems may have spatially inhomogeneous stationary solutions called Turing patterns. These solutions are typically non-unique and it is not clear how many of them exists. Since there are no analytical results available, we look for the number of distinct stationary solutions numerically. As a typical example, we investigate the reaction-diffusion systém designed to model coat patterns in leopard and jaguar.
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Irregularity of turing patterns in the Thomas model with a unilateral term
Rybář, Vojtěch ; Vejchodský, Tomáš
In this contribution we add a unilateral term to the Thomas model and investigate the resulting Turing patterns. We show that the unilateral term yields nonsymmetric and irregular patterns. This contrasts with the approximately symmetric and regular patterns of the classical Thomas model. In addition, the unilateral term yields Turing patterns even for smaller ratio of diffusion constants. These conclusions accord with the recent findings about the influence of the unilateral term in a model for mammalian coat patterns. This indicates that the observed effects of the unilateral term are general and apply to a variety of systems.
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A direct solver for finite element matrices requiring O(N log N) memory places
Vejchodský, Tomáš
We present a method that in certain sense stores the inverse of the stiffness matrix in O(N log N) memory places, where N is the number of degrees of freedom and hence the matrix size. The setup of this storage format requires O(N^(3/2)) arithmetic operations. However, once the setup is done, the multiplication of the inverse matrix and a vector can be performed with O(N log N) operations. This approach applies to the first order finite element discretization of linear elliptic and parabolic problems in triangular domains, but it can be generalized to higher-order elements, variety of problems, and general domains. The method is based on a special hierarchical enumeration of vertices and on a hierarchical elimination of suitable degrees of freedom. Therefore, we call it hierarchical condensation of degrees of freedom.
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Guaranteed and fully computable two-sided bounds of Friedrichs' constant
Vejchodský, Tomáš
This contribution presents a general numerical method for computing lower and upper bound of the optimal constant in Friedrichs’ inequality. The standard Rayleigh-Ritz method is used for the lower bound and the method of a priori-a posteriori inequalities is employed for the upper bound. Several numerical experiments show applicability and accuracy of this approach.
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Computing upper bounds on Friedrichs' constant
Vejchodský, Tomáš
This contribution shows how to compute upper bounds of the optimal constant in Friedrichs’ and similar inequalities. The approach is based on the method of a prioria posteriori inequalities [9]. However, this method requires trial and test functions with continuous second derivatives. We show how to avoid this requirement and how to compute the bounds on Friedrichs’ constant using standard finite element methods. This approach is quite general and allows variable coefficients and mixed boundary conditions. We use the computed upper bound on Friedrichs’ constant in a posteriori error estimation to obtain guaranteed error bounds.
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Programs and Algorithms of Numerical Mathematics 15
Vejchodský, Tomáš ; Chleboun, J. ; Přikryl, Petr ; Segeth, Karel ; Šístek, Jakub
The book contains papers presented at the international seminar Programs and algorithms of numerical Mathematics 15 (PANM 15), held in Dolni Maxov, Czech Republic, June 6-11, 2010. It is the fifteen volume in the series of the PANM proceedings. The topics of contributions include numerical methods for fluid flow modelling, the finite element method, a posteriori error estimates, topics from numerical linear atgebra, etc.
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Complementarity - the way towards guaranteed error estimates
Vejchodský, Tomáš
This paper presents a review of the complementary technique with the emphasis on computable and guaranteed upper bounds of the approximation error. For simplicity, the approach is described on a numerical solution of the Poisson problem. We derive the complementary error bounds, prove their fundamentals properties, present the method of hypercircle, mention possible generalizations and show a couple of numerical examples.
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Programy a algoritmy numerické matematiky 14
Chleboun, Jan ; Přikryl, Petr ; Segeth, Karel ; Vejchodský, Tomáš
Kniha obsahuje příspěvky příspěvky přednášek na mezinárodním semináři Programy a algoritmy numerické matematiky 14 (PANM 14) konaném v Dolním Maxově ve dnech 1.-6. červnas 2008. Jde o čtrnáctou publikaci v sérii sborníku PANM. Příspěvky se zaměřují na numerické metody a na proudění tekutin, metodu konečných prvků, aposteriorní odhady chyby, témata z numerické lineární algebry a podobně.
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Výpočetní srovnání diskretizační a iterační chyby
Vejchodský, Tomáš
Článek prezentuje Poissonovu rovnici a její řešení pomocí metody konečných prvků. Ukazuje se numerický příklad, u kterého je explicitně známo kromě přesného řešení také přesné diskrétní řešení. Díky tomu je možné porovnávat diskretizační chyby a chybu vzniklou přibližným řešením soustavy lineárních algebraických rovnic.
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