655 research outputs found
Solving AX-equations
No full textTitle: Solving AX-equations Author: Jan Butora Department: Department of algebra Supervisor: doc. RNDr. Jiří Tůma, DrSc., Department of algebra Abstract: In this work, we present concept of AX-equations and focus on two such equations. Using similiar techniques, we build a theory for both equations, which allows us to express number of their solutions based only on their parameters. Using this theory, we demonstrate on an example that differential steps, used in differential cryptanalysis of modular addition, are not independent. Moreover, based on this theory we introduce and implement fast algorithms for searching solutions. Keywords: differential cryptanalysis, AX-equations, modular addition, carry, sol- vability conditio
Theoretical investigation on radiation tolerance of Mn+1 AX(n) phases
No full textTernary M(n + 1)AX(n) phases with layered hexagonal structures, as candidate materials used for next-generation nuclear reactors, have shown great potential in tolerating radiation damage due to their unique combination of ceramic and metallic properties. However, Mn + 1AXn materials behave differently in amorphization when exposed to energetic neutron and ion irradiations in experiment. We first analyze the irradiation tolerances of different M(n + 1)AX(n) (MAX) phases in terms of electronic structure, including the density of states ( DOS) and charge density map. Then a new method based on the Bader analysis with the first-principle calculation is used to estimate the stabilities of MAX phases under irradiation. Our calculations show that the substitution of Cr/V/Ta/Nb by Ti and Si/Ge/Ga by Al can increase the ionicities of the bonds, thus strengthening the radiation tolerance. It is also shown that there is no obvious difference in radiation tolerance between Mn (+ 1)AC(n) and Mn (+ 1)AN(n) due to the similar charge transfer values of C and N atoms. In addition, the improved radiation tolerance from Ti3AlC2 to Ti2AlC ( Ti3AlC2 and Ti2AlC have the same chemical elements), can be understood in terms of the increased Al/TiC layer ratio. Criteria based on the quantified charge transfer can be further used to explore other M(n + 1)AX(n) phases with respect to their radiation tolerance, playing a critical role in choosing appropriate MAX phases before they are subjected to irradiation in experimental test for future nuclear reactors
Ab initio study of irradiation tolerance for different M(n+1)AX(n) phases: Ti3SiC2 and Ti3AlC2
No full textLayered ternary M(n+1)AX(n) (MAX) materials are recently proposed to be promising candidates for future fission and fusion programmes because of their unique properties inherited from both ceramics and metals. However, different M(n+1)AX(n) materials demonstrate different behaviors when exposed to energetic neutron or ion irradiations. Based on first-principles calculations, we have investigated the irradiation tolerance of two typical M(n+1)AX(n) materials: Ti3SiC2 and Ti3AlC2 from two aspects. First, we make a detailed analysis on the interatomic bonding characters, which are believed to be responsible for the resistance to radiation-induced amorphization. Second, the formation energies of various intrinsic and antisite defects in these two compounds are calculated in order to elucidate their amorphization mechanism. Our results show that the absence of orbitals overlap of Al-C in Ti3AlC2 renders it more resistant to amorphization compared to Ti3SiC2. In addition, the antisite defects Al-Ti(1) and Al-Ti(2) in Ti3AlC2 have much lower formation energies compared to Si-Ti(1) and Si-Ti(2) in Ti3SiC2, which implies that the replacement of Ti with Al is easier than Si, thus providing an alternative way to accommodate the defects resulted from irradiation damage cascades. These results indicate that Ti3AlC2 is more irradiation tolerant than Ti3SiC2, in accordance with experimental observations. Our results have profound implications for the choice of appropriate MAX phase with best performance to be used in next reaction reactors. (C) 2014 AIP Publishing LLC
Robust Pole Assignment via Sylvester Equation Based State Feedback Parametrization
No full textBy using a Sylvester equation based parametrization, the minimum norm robust pole assignment problem for linear time-invariant systems is formulated as an unconstrained minimization problem for a suitably chosen cost function. The derived explicit expression of the gradient of the cost function allows the efficient solution of the minimization problem by using powerful gradient search based minimization techniques. We also discuss how requirements for a particular Jordan structure of the closed-loop state matrix or for partial pole asignment can be accomodated with the proposed approach
Minding the aesthetic: The place of the literary in education and research.
No full textThe article discusses the significance of aesthetic as a mode of cognition and means of social cohesion. It notes the relation of aesthetic knowledge with the perception or intuition, the emergence of such awareness into something durable and the response to the embodiment. It describes the evolution of aesthetic delight in the human species, the sense of sense of beauty arising on one's realization of the formal qualities of something, through the poem presented by the author on achievement
ko-ax photo [Selected by Tate Curator of Photography, Simon Baker]
No full textko-ax photo was an open submission competition selected by Simon Baker (Curator of Photography, Tate), Sue Steward (Photography critic, Telegraph, Observer, Guardian, BBC) and John Gill (Curator Brighton Photo Biennial and Founder Photoworks). The 10 artists selected all presented fascinating artworks that conceal narratives and ask questions of the viewer. Questions of beauty, family, decay and fantasy were all explored across over 50 works.</p
Preconditioning Large Scale Iterative Solution of Ax = b Using a Statistical Method with Application to Matrix-Free Spectral Solution of Helmholtz Equation
No full textAbstractThe problem of preconditioning the linear system Ax = b where no entries of A are known but the matrix-vector product is given in the linear functional form Ax = L(x) is considered. A statistical multiregressor method is proposed whose convergence improves during Krylov iterations. It is shown that this method effectively improves the rate of convergence of the GMRES algorithm. The results are validated by applying the current preconditioner to a pseudo-spectral solution of Helmholtz equation
Robust pole assignment techniques via state feedback
No full textWe present a unifying computational framework to solve robust pole assignment problems for linear systems using state feedback. The new framework uses Sylvester equation based parametrizations of the pole assignment problems. The non-uniqueness of solutions is exploited by minimizing additionally sensitivity of closed-loop eigenvalues and the norm of the corresponding state feedback matrix. The solution methods rely on using gradient search based minimization techniques on suitably defined cost functions. The discussion of main functional and numerical aspects reveals many desirable features of the underlying algorithms which recommend them to serve as bases for robust numerical software implementations
Veilig Getij, beheer en gebruik Stormvloedkering Oosterschelde: Ervaring en bijstelling
No full textDe rapportage van het project Evaluatie Oosterschelde bestaat uit de evaluatienota 'de evaluatie van de Oosterschelde na 5 jaar stormvloedkering' (AX91.089) en twee onderbouwende nota's: 'Beheer stormvloedkering Oosterschelde: ervaring en bijstelling' en 'De effecten van de waterbouwkundige werken op het getijdemilieu van de Oosterschelde'(AX91.091; GWWS91.088). Deze nota is gebaseerd op onderzoek dat is uitgevoerd in de periode 1987-1991. Het onderzoek bestond enerzijds uit veldwaarnemingen anderzijds uit nader theoretisch onderzoek betreffende de door de projektgroep BARCON ten tijde de aanleg van de kering geformuleerde uitgangspunten. Bij de interpretatie van de gegevens is gebruik gemaakt van de nieuwste wetenschappelijke inzichten. Tevens is gebruik gemaakt van nieuwe rekentechnieken. Het onderzoek is uitgevoerd in opdracht van direktie Zeeland van Rijkswaterstaat.KWP-collectio
A constructive proof of a unimodular transformation theorem for simplices
No full textMatrices;Linear Programming;Integer Programming
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