Within the framework of a project sponsored by the European Space Agency (ESA), we have developed a software tool to predict the occurrence of multipactor discharge in a simple radio frequency (RF) device modeled as parallel plates. The tool uses a micro-level explicit representation of the electrons (i.e., each electron in the system is modeled separately), and includes a detailed Monte Carlo model of the secondary electron emission process in the plates. Materials secondary emission yield (SEY) is described using either the usual parameter set (E/sub 1/, E/sub 2/, and /spl sigma//sub max/), or a more detailed model, where the contributions due to true secondary, backscattered or elastically reflected electrons are given their own sets of parameters, together with additional parameters for the angle dependence. The simulator has been validated using experimental data gathered at ESA and the Universidad Auto/spl acute/noma de Madrid, Madrid, Spain. The simulator helped in the selection of material coatings for the mitigation of Multipactor effect in RF transmission lines on-board satellite payloads.

Using the mathematical background for algorithmic complexity developed by Kolmogorov in the sixties, Cilibrasi and Vitanyi have designed a similarity distance named normalized compression distance applicable to the clustering of objects of any kind, such as music, texts or gene sequences. The normalized compression distance is a quasi-universal normalized admissible distance under certain conditions. This paper shows that the compressors used to compute the normalized compression distance are not idempotent in some cases, being strongly skewed with the size of the objects and window size, and therefore causing a deviation in the identity property of the distance if we don't take care that the objects to be compressed fit the windows. The relationship underlying the precision of the distance and the size of the objects has been analyzed for several well-known compressors, and specially in depth for three cases, bzip2, gzip and PPMZ which are examples of the three main types of compressors: block-sorting, Lempel-Ziv, and statistic.This work was partially supported by grant TSI 2005- 08255-C07-06 of the Spanish Ministry of Education and Science

This correspondence studies the influence of noise on the normalized compression distance (NCD), a measure based on the use of compressors to compute the degree of similarity of two files. This influence is approximated by a first order differential equation which gives rise to a complex effect, which explains the fact that the NCD may give values greater than 1, observed by other authors. The model is tested experimentally with good adjustment. Finally, the influence of noise on the clustering of files of different types is explored, finding that the NCD performs well even in the presence of quite high noise levels

A digital image scrambling method based on a 2D cellular automaton, specifically the well-known Game of Life, produces an effective image encryption technique.

This paper describes Christiansen grammar evolution (CGE), a new evolutionary automatic programming algorithm that extends standard grammar evolution (GE) by replacing context-free grammars by Christiansen grammars. GE only takes into account syntactic restrictions to generate valid individuals. CGE adds semantics to ensure that both semantically and syntactically valid individuals are generated. It is empirically shown that our approach improves GE performance and even allows the solution of some problems are difficult to tackle by GE