3,123 research outputs found
Higher Order Analogues of Tracy-Widom Distributions via the Lax Method
No full textAkemann G, Atkin M. Higher Order Analogues of Tracy-Widom Distributions via the Lax Method. J.Phys. A. 2013;46(1): 15202.We study the distribution of the largest eigenvalue in formal Hermitianone-matrix models at multicriticality, where the spectral density acquires anextra number of k-1 zeros at the edge. The distributions are directly expressedthrough the norms of orthogonal polynomials on a semi-infinite interval, as analternative to using Fredholm determinants. They satisfy non-linear recurrencerelations which we show form a Lax pair, making contact to the stringliterature in the early 1990's. The technique of pseudo-differential operatorsallows us to give compact expressions for the logarithm of the gap probabilityin terms of the Painleve XXXIV hierarchy. These are the higher order analoguesof the Tracy-Widom distribution which has k=1. Using known Backlundtransformations we show how to simplify earlier equivalent results that arederived from Fredholm determinant theory, valid for even k in terms of thePainleve II hierarchy
CDT coupled to dimer matter: An analytical approach via tree bijections
No full textAtkin M, Zohren S. CDT coupled to dimer matter: An analytical approach via tree bijections. AIP Conference Proceedings. 2012;1483(1):330-335.We review a recently obtained analytical solution of a restricted so-calledhard dimers model coupled to two-dimensional CDT. The combinatorial solution isobtained via bijections of causal triangulations with dimers and decoratedtrees. We show that the scaling limit of this model can also be obtained from amulti-critical point of the transfer matrix for dynamical triangulations oftriangles and squares when one disallows for spatial topology changes to occur
A Transfer Report on the Development of a Framework to Evaluate Search Interfaces for their Support of Different User Types and Search Tactics
No full textAs the understanding of search systems, user needs and seeking strategies is developing, the design of search user interfaces is evolving to support more complicated and exploratory forms of search. With the design of new search features that enable these richer modes of exploration, comes the need to better understand the support they provide. In this report a new evaluation framework is presented that analyses search features for how they a) contribute to an overall interface, b) allow users to carry out different search tactics, and c) support different types of users and their needs. The novel contributions of the framework improve on some of the limitations of typical user studies, and allow search systems to be systematically analysed in much more detail and in much less time. The presented evaluation framework is then validated in three ways. First the validity of the models used as the building blocks of the framework are investigated through related work. Second the method of integrating these building-block models is validated and strengthened by consensus of expert opinion. Third, the overall approach is validated by comparing its analyses to the results of previously carried out user studies. The validation process has shown both the value of the framework and identified areas of future work that should be addressed for the framework to be completed. This report concludes with the set of contributions that the framework makes, and why the remaining work will be challenging, but critical to the final design
An analytical analysis of CDT coupled to dimer-like matter
No full textAtkin MR, Zohren S. An analytical analysis of CDT coupled to dimer-like matter. Physics Letters B. 2012;712(4-5):445-450.We consider a model of restricted dimers coupled to two-dimensional Causal Dynamical Triangulations (CDT), where the dimer configurations are restricted in the sense that they do not include dimers in regions of high curvature. It is shown how the model can be solved analytically using bijections with decorated trees. At a negative critical value for the dimer fugacity the model undergoes a phase transition at which the critical exponent associated to the geometry changes. This represents the first account of an analytical study of a matter model with two-dimensional interactions coupled to CDT. (C) 2012 Elsevier B.V. All rights reserved
Instantons and Extreme Value Statistics of Random Matrices
No full textAtkin M, Zohren S. Instantons and Extreme Value Statistics of Random Matrices. Journal of High Energy Physics. 2014;2014(4): 118.We discuss the distribution of the largest eigenvalue of a random N x NHermitian matrix. Utilising results from the quantum gravity and string theoryliterature it is seen that the orthogonal polynomials approach, firstintroduced by Majumdar and Nadal, can be extended to calculate both the leftand right tail large deviations of the maximum eigenvalue. This framework doesnot only provide computational advantages when considering the left and righttail large deviations for general potentials, as is done explicitly for thefirst multi-critical potential, but it also offers an interestinginterpretation of the results. In particular, it is seen that the left taillarge deviations follow from a standard perturbative large N expansion of thefree energy, while the right tail large deviations are related to thenon-perturbative expansion and thus to instanton corrections. Considering thestandard interpretation of instantons as tunnelling of eigenvalues, we see thatthe right tail rate function can be identified with the instanton action whichin turn can be given as a simple expression in terms of the spectral curve.From the string theory point of view these non-perturbative correctionscorrespond to branes and can be identified with FZZT branes
Die Bestimmung von Forschungsthemen in Max-Planck-Instituten im Spannungsfeld wissenschaftlicher und außerwissenschaftlicher Interessen: Ein Forschungsbericht
No full textHeute steht im Zentrum der öffentlichen Diskussion die Frage, wie sich der praktische Nutzen der Wissenschaft für Wirtschaft und Gesellschaft maximieren lässt. Die empirische Untersuchung, deren Ergebnisse in diesem Bericht dargestellt werden, geht von der umgekehrten Frage aus: Wie beeinflusst die gesellschaftliche Einbettung von Forschungseinrichtungen, vermittelt über die Auswahl von Forschungsthemen, die Entwicklung der Wissenschaft? Gegenstand der Untersuchung waren Institute der Max-Planck-Gesellschaft. Zunächst wurden statistische Daten über die Außenbeziehungen von Max-Planck-Instituten, die die Generalverwaltung der MPG zur Verfügung stellte, analysiert. In den Jahren 1999 und 2000 führte die Verfasserin Interviews mit den Direktoren ausgewählter Institute aller drei Sektionen der MPG durch und wertete verfügbare schriftliche Unterlagen über ihre Forschungstätigkeit aus. Der Bericht schildert exemplarisch den Prozess der Bestimmung von Forschungsthemen in Max-Planck-Instituten und wie dieser vom institutionellen Rahmen, von den Beziehungen zur wissenschaftlichen Fachwelt und von den Beziehungen zu verschiedenen Praxisfeldern beeinflusst wird. In Abhängigkeit von der Eigenart der Forschungsfelder, in denen ein Institut tätig ist, und dessen potentieller Praxisrelevanz ergeben sich verschiedene Muster von Außenbeziehungen, in denen je nachdem die Beziehungen zur Praxis oder die Beziehungen zur wissenschaftlichen Fachwelt größere Bedeutung für die Themenwahl haben.Today the usefulness of science for society and economy is the focal issue in science policy. This report presents the results of an empirical study starting from a different perspective, i.e. how the societal embeddedness of research institutions affects the choice of research topics and thus, in the aggregate, the direction of scientific development. In this study, Max Planck institutes were the object of investigation. In a first phase, statistical data about the external relations of Max Planck institutes were analyzed. In 1999 and 2000, this was followed by a series of interviews which the author conducted with the directors of selected institutes, supplemented by the analysis of written documents. The report shows in an exemplary fashion how the process of choosing research topics in Max Planck institutes is affected by their institutional shape, their interaction with different fields of societal praxis, and their relations to the national and international scientific communities. The resulting patterns of external relations and the relative importance of links with the scientific community and of links with fields of (e.g. industrial, medical) praxis differ according to the potential applicability of given research fields
Identification of a chlorodibenzo-p-dioxin dechlorinating Dehalococcoides mccartyi by stable isotope probing
No full textPolychlorinated dibenzo-p-dioxins (PCDDs) are released into the environment from a variety of both anthropogenic and natural sources. While highly chlorinated dibenzo-p-dioxins are persistent under oxic conditions, in anoxic environments these organohalogens can be reductively dechlorinated to less chlorinated compounds that are then more amenable to subsequent aerobic degradation. Identifying the microorganisms responsible for dechlorination is an important step in developing bioremediation approaches. In this study, we demonstrated the use of a DNA-stable isotope probing (SIP) approach to identify the bacteria active in dechlorination of PCDDs in river sediments, with 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD) as a model. In addition, pyrosequencing of reverse transcribed 16S rRNA of TeCDD dechlorinating enrichment cultures was used to reveal active members of the bacterial community. A set of operational taxonomic units (OTUs) responded positively to the addition of 1,2,3,4-TeCDD in SIP microcosms assimilating 13C-acetate as the carbon source. Analysis of bacterial community profiles of the 13C labeled heavy DNA fraction revealed that an OTU corresponding to Dehalococcoides mccartyi accounted for a significantly greater abundance in cultures amended with 1,2,3,4-TeCDD than in cultures without 1,2,3,4-TeCDD. This implies the involvement of this Dehalococcoides mccartyi strain in the reductive dechlorination of 1,2,3,4-TeCDD, and suggests the applicability of SIP for a robust assessment of the bioremediation potential of organohalogen contaminated sites.Peer reviewe
Optimal solution of the nearest correlation matrix problem by minimization of the maximum norm
No full textThe nearest correlation matrix problem is to find a valid (positive semidefinite) correlation matrix, R(m,m), that is nearest to a given invalid (negative semidefinite) or pseudo-correlation matrix, Q(m,m); m larger than 2. In the literature on this problem, 'nearest' is invariably defined in the sense of the least Frobenius norm. Research works of Rebonato and Jaeckel (1999), Higham (2002), Anjos et al. (2003), Grubisic and Pietersz (2004), Pietersz, and Groenen (2004), etc. use Frobenius norm explicitly or implicitly. However, it is not necessary to define 'nearest' in this conventional sense. The thrust of this paper is to define 'nearest' in the sense of the least maximum norm (LMN) of the deviation matrix (R-Q), and to obtain R nearest to Q. The LMN provides the overall minimum range of deviation of the elements of R from those of Q. We also append a computer program (source codes in FORTRAN) to find the LMN R from a given Q. Presently we use the random walk search method for optimization. However, we suggest that more efficient methods based on the Genetic algorithms may replace the random walk algorithm of optimization.Nearest correlation matrix problem; Frobenius norm; maximum norm; LMN correlation matrix; positive semidefinite; negative semidefinite; positive definite; random walk algorithm; Genetic algorithm; computer program; source codes; FORTRAN; simulation
FZZT Brane Relations in the Presence of Boundary Magnetic Fields
No full textAtkin M, Zohren S. FZZT Brane Relations in the Presence of Boundary Magnetic Fields. Journal of High Energy Physics. 2012;2012(11): 163.We show how a boundary state different from the (1,1) Cardy state may berealised in the (m,m+1) minimal string by the introduction of an auxiliarymatrix into the standard two hermitian matrix model. This boundary is a naturalgeneralisation of the free spin boundary state in the Ising model. Theresolvent for the auxiliary matrix is computed using an extension of thesaddle-point method of Zinn-Justin to the case of non-identical potentials. Thestructure of the saddle-point equations result in a Seiberg-Shih like relationbetween the boundary states which is valid away from the continuum limit, inaddition to an expression for the spectral curve of the free spin boundarystate. We then show how the technique may be used to analyse boundary statescorresponding to a boundary magnetic field, thereby allowing us to generalisethe work of Carroll et al. on the boundary renormalisation flow of the Isingmodel, to any (m,m+1) model
Arctic tundra soil bacterial communities active at subzero temperatures detected by stable isotope probing
No full textArctic soils store vast amounts of carbon and are subject to intense climate change. While effects of thaw on the composition and activities of Arctic tundra microorganisms has been examined extensively, little is known about the consequences of temperature fluctuations within the subzero range in seasonally frozen or permafrost soils. This study identified tundra soil bacteria active at subzero temperatures using stable isotope probing (SIP). Soils from Kilpisjärvi, Finland were amended with 13C-cellobiose and incubated at 0, -4, and -16°C for up to 40 weeks. 16S rRNA gene sequence analysis of 13C-labelled DNA revealed distinct subzero-active bacterial taxa. The SIP experiments demonstrated that diverse bacteria, including members of Candidatus Saccharibacteria, Melioribacteraceae, Verrucomicrobiaceae, Burkholderiaceae, Acetobacteraceae, Armatimonadaceae, and Planctomycetaceae were capable of synthesizing 13C-DNA at subzero temperatures. Differences in subzero temperature optima were observed, for example with members of Oxalobacteraceae and Rhizobiaceae found to be more active at 0°C than at -4°C or -16°C, whereas Melioribacteriaceae were active at all subzero temperatures tested. Phylogeny of 13C-labelled 16S rRNA genes from the Melioribacteriaceae, Verrucomicrobiaceae, and Candidatus Saccharibacteria suggested that these taxa formed subzero-active clusters closely related to members from other cryo-environments. This study demonstrates that subzero temperatures impact active bacterial community composition and activity which may influence biogeochemical cycles.Peer reviewe
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