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1Author    Heidrun Amm, Maren Grigutsch, Ralf StannariusRequires cookie*
 Title    Spatio-temporal Analysis of Electroconvection in Nematics  
 Abstract    We investigate electroconvection in nematic liquid crystals by means of optical microscopy. Time resolved optical images are used to study the director dynamics. For the first time we present instant images in the dielectric regime. A numerical simulation of the optical transmission patterns is performed on the basis of Fermat's principle. In the instant images of dielectric rolls, the periodicity of the observed optical pattern is equal to the wavelength Ao of the convection rolls. The well known low contrast 'stationary' optical texture observed in conventional experiments results from time averaging of these instant images; its wavelength is Ao/2. 
  Reference    Z. Naturforsch. 53a, 117—126 (1998); received December 10 1997 
  Published    1998 
  Keywords    Liquid Crystals, Pattern Formation, Electrohydrodynamic Instability, Optical Microscopy 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0117.pdf 
 Identifier    ZNA-1998-53a-0117 
 Volume    53 
2Author    J. Ackermann, T. KirnerRequires cookie*
 Title    Parasites and Pattern Formation  
 Abstract    Biological information is coded in replicating molecules. To maintain a given amount of in-formation a cooperative interaction between these molecules is essential. The main problem for the stability of a system of prebiotic replicators are emerging parasites. Stabilization against such parasites is possible if space is introduced in the model. Complex patterns like spiral waves and self-replicating spot patterns have been shown to stabilize such systems. Stability of replicating systems, however, occurs only in parameter regions were such complex patterns occur. We show that parasites are able to push such systems into a parameter region were life is possible. To demonstrate this influence of parasites on such systems, we introduce a parasitic species in the Gray-Scott model. The growing concentration of parasites will kill the system, and the cooperative Gray-Scott system will be diluted out in a well mixed flow reactor. While considering space, in the model stabilizing pattern formation in a narrow parameter region is possible. We demonstrate that the concentration of the parasitic species is able to push the system into a region were stabilizing patterns emerge. 
  Reference    Z. Naturforsch. 54a, 146—152 (1999); received December 15 1998 
  Published    1999 
  Keywords    Pattern Formation, Reaction-Diffusion, Molecular Evolution, Parasites, Coevolution, Cooperation 
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 TEI-XML for    default:Reihe_A/54/ZNA-1999-54a-0146.pdf 
 Identifier    ZNA-1999-54a-0146 
 Volume    54 
3Author    HansH. Diebner3, O. Tf\, Adolf Mathias3, Horst Prehnc, Marco Rohrbacha, Sven Sahle3Requires cookie*
 Title    Control and Adaptation of Spatio-temporal Patterns  
 Abstract    We apply a recently introduced cognitive system for dynamics recognition to a two-dimensional array of coupled oscillators. The cognitive system allows for both the control and the adaptation of spatio-temporal patterns of that array of oscillators. One array that shows Turing-pattems in a self-organizational manner is viewed as an externally presented dynamics (stimulus) which is mapped onto a mirror dynamics, whereby the latter is capable to simulate (simulus). Two of the parameters of the stimulus are thereby regarded to be unknown and have to be estimated by the cognitive system. The cognitive system itself consists of dynamical modules that are stimulated by the external dynamics in the sense of Pyragas' external force control mechanism and thereby yield measures of how good they match the stimulus. These measures are used as weights to construct the simulus. The adaptation process is performed "on the fly", i. e., without the storage of data. The proposed cognitive system, therefore, is a prominent candidate for the construction of a control device for a permanent real time observation of an external dynamical system in order to interfere instantaneously when necessary. 
  Reference    Z. Naturforsch. 56a, 663—669 (2001); received August 21 2001 
  Published    2001 
  Keywords    Adaptive Systems, Cognitive Systems, Pattern Formation, Brain Dynamics, Force Control 
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 TEI-XML for    default:Reihe_A/56/ZNA-2001-56a-0663.pdf 
 Identifier    ZNA-2001-56a-0663 
 Volume    56