1,720,968 research outputs found
Krylov methods for low-rank commuting generalized Sylvester equations
Full text linkWe consider generalizations of the Sylvester matrix equation, consisting of the sum of a Sylvester operator and a linear operator pi with a particular structure. More precisely, the commutators of the matrix coefficients of the operator pi and the Sylvester operator coefficients are assumed to be matrices with low rank. We show (under certain additional conditions) low-rank approximability of this problem, that is, the solution to this matrix equation can be approximated with a low-rank matrix. Projection methods have successfully been used to solve other matrix equations with low-rank approximability. We propose a new projection method for this class of matrix equations. The choice of the subspace is a crucial ingredient for any projection method for matrix equations. Our method is based on an adaption and extension of the extended Krylov subspace method for Sylvester equations. A constructive choice of the starting vector/block is derived from the low-rank commutators. We illustrate the effectiveness of our method by solving large-scale matrix equations arising from applications in control theory and the discretization of PDEs. The advantages of our approach in comparison to other methods are also illustrated.QC 20181127</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Algoritmer med lag komplexitet for snabbare-an-Nyquist signalering
No full textThis thesis is an investigation of what happens when communication links are pushed towards their limits and the data-bearing-pulses are packed tighter in time than previously done. This is called faster-than-Nyquist (FTN) signaling and it will violate the Nyquist inter-symbol interference criterion, implying that the data-pulsesare no longer orthogonal and thus that the samples at the receiver will be dependent on more than one of the transmitted symbols. Inter-symbol interference (ISI) has occurred and the consequences of it are studied for the AWGN-channel model. Here it is shown that in order to do maximum likelihood estimation on these samples the receiver will face an NP-hard problem. The standard algorithm to make good estimations in the ISI case is the Viterbi algorithm, but applied on a block with N bits and interference among K bits thecomplexity is O(N *2K), hence limiting the practical applicability. Here, a precoding scheme is proposed together with a decoding that reduce the estimation complexity. By applying the proposed precoding/decoding to a data block of length N the estimation can be done in O(N2) operations preceded by a single off-line O(N3) calculation. The precoding itself is also done in O(N2)operations, with a single o ff-line operation of O(N3) complexity. The strength of the precoding is shown in simulations. In the first it was tested together with turbo codes of code rate 2/3 and block lengthof 6000 bits. When sending 25% more data (FTN) the non-precoded case needed about 2.5 dB higher signal-to-noise ratio (SNR) to have the same error rate as the precoded case. When the precoded case performed without any block errors, the non-precoded case still had a block error rate almost equal to 1. We also studied the scenario of transmission with low latency and high reliability. Here, 600 bits were transmitted with a code rate of 2/3, and hence the target was to communicate 400 bits of data. Applying FTN with doublepacking, that is transmitting 1200 bits during the same amount of time, it was possible to lower the code rate to 1/3 since only 400 bits of data was to be communicated. This technique greatly improves the robustness. When the FTN case performed error free, the classical Nyquist case still had a block error rate of 0.19. To reach error free performance the Nyquist case needed 1.25 dB higher SNR compared to the precoded FTN case with lower code rate.Detta examensarbete handlar om vad som händer då kommunikationskanaler pressas till sin gräns och pulserna som bär data packas tätare i tiden. Detta kallas snabbare-än-Nyquist (FTN) och kommer att bryta mot Nyquists kriterium för intersymbolinterferens, vilket innebär att de databärande pulserna inte längre kommer vara ortogonala och att signalsamplen kommer vara beroende av mer än en skickad symbol. Det uppstår intersymbolinterferens (ISI) och dess konsekvenser studeras inom kanalmodellen AWGN. Vi visar att göra en maximum likelihood uppskattning baserat på dessa data är ett NP-svårt problem. Normalt används Viterbi algoritmen när man har ISI, men den har exponentiell komplexitet. På ett block med N symboler och interferens i storleken K symboler är komplexiteten O(N*2K) vilket gör att algoritmen är svår att använda i praktiska fall. Istället så föreslås en förkodning, som tillsammans med en avkodning reducerar komplexiteten. Kodningen appliceras blockvis och på ett block med N symboler är komplexiteten O(N2) för kodning/avkodning. Denna måste i båda fall föregås av en O(N3) beräkning, som dock behöver göras endast en gång. Simuleringar visar den föreslagna kodningens fördelar. I den första simuleringen testades den ihop med turbokodning med blocklängd på 6000 bitar och en kodningsgrad på 2/3. När FTN användes för att skicka 25% mer data krävdes det cirka 2.5 dB högre signal-till-brus-förhållande (SNR) för att den icke förkodade signalen skulle ha samma felfrekvens som den förkodade. När det förkodade fallet presterade felfritt gjorde det oförkodade fel på nästan alla block. Ett annat scenario som testades var det med korta koder, liten fördröjning och hög robusthet. I detta scenario skickades 600 bitar med en kodningsgrad på 2/3, alltså 400 bitar ren data. Genom att använda FTN med en dubbel packningsgrad, vilket innebär att 1200 bitar skickades under samma tid, var det möjligt att sänka kodningsgraden till 1/3, eftersom det bara var 400 bitar ren data som skulle överföras. Detta ökad robustheten i systemet ty då FTN fallet gjorde felfritt hade det klassiska Nyquist fallet fortfarande en felfrekvens på 0.19 för sina block. Det krävdes 1.25 dB högre SNR för Nyquist fallet att bli felfritt jämfört med FTN och lägre kodningsgrad
Numerical methods for Sylvester-type matrix equations and nonlinear eigenvalue problems
No full textLinear matrix equations and nonlinear eigenvalue problems (NEP) appear in a wide variety of applications in science and engineering. Important special cases of the former are the Lyapunov equation, the Sylvester equation, and their respective generalizations. These appear, e.g., as Gramians to linear and bilinear systems, in computations involving block-triangularization of matrices, and in connection with discretizations of some partial differential equations. The NEP appear, e.g., in stability analysis of time-delay systems, and as results of transformations of linear eigenvalue problems. This thesis mainly consists of 4 papers that treats the above mentioned computational problems, and presents both theory and methods. In paper A we consider a NEP stemming from the discretization of a partial differential equation describing wave propagation in a waveguide. Some NEP-methods require in each iteration to solve a linear system with a fixed matrix, but different right-hand sides, and with a fine discretization, this linear solve becomes the bottleneck. To overcome this we present a Sylvester-based preconditioner, exploiting the Sherman–Morrison–Woodbury formula. Paper B treats the generalized Sylvester equation and present two main results: First, a characterization that under certain assumptions motivates the existence of low-rank solutions. Second, a Krylov method applicable when the matrix coefficients are low-rank commuting, i.e., when the commutator is of low rank. In Paper C we study the generalized Lyapunov equation. Specifically, we extend the motivation for applying the alternating linear scheme (ALS) method, from the stable Lyapunov equation to the stable generalized Lyapunov equation. Moreover, we show connections to H2-optimal model reduction of associated bilinear systems, and show that ALS can be understood to construct a rank-1 model reduction subspace to such a bilinear system related to the residual. We also propose a residual-based generalized rational-Krylov-type subspace as a solver for the generalized Lyapunov equation. The fourth paper, Paper D, connects the NEP to the two-parameter eigenvalue problem. The latter is a generalization of the linear eigenvalue problem in the sense that there are two eigenvalue-eigenvector equations, both depending on two scalar variables. If we fix one of the variables, then we can use one of the equations, which is then a generalized eigenvalue problem, to solve for the other variable. In that sense, the solved-for variable can be understood as a family of functions of the first variable. Hence, it is a variable elimination technique where the second equation can be understood as a family of NEPs. Methods for NEPs can thus be adapted and exploited to solve the original problem. The idea can also be reversed, providing linearizations for certain NEPs.Linjära matrisekvationer är en vanligt förekommande variant av linjära ekvationssystem. Viktiga specialfall är Lyapunovekvationen och Sylvesterekvationen, samt deras respektive generaliseringar. Dessa ekvationer uppstår till exempel som karakteriseringar av Gramianer till linjära och bilinjära dynamiska system, i beräkningar som innebär blocktriangularisering av matriser, och vid diskretiseringar av vissa partiella differentialekvationer. Det icke-linjära egenvärdesproblemet, från engelskan förkortat NEP, är en generalisering av det linjära egenvärdesproblemet för en matris. I det icke-linjära fallet tillåts matrisens beroende på den skalära parametern att vara just icke-linjärt. Formellt betraktas problemet som en funktion vars definitionsmängd är en delmängd av de komplexa talen, och vars värdemängd är (en delmängd av de) komplexvärda matriserna. Problemet kan beskrivas som att hitta värden, så kallade egenvärden, som gör att den tillhörande matrisen i värdemängden är singulär; en vektor i nollrummet kallas för en egenvektor. Notera att beroendet på egenvektorn är linjärt. Tillämpningar inkluderar bland annat studier av dynamiska system med tidsfördröjning, samt vid transformationer av linjära egenvärdesproblem. En annan generalisering av egenvärdesproblemet är två-parameters egenvärdesproblemet vilket består av två matrisvärda funktioner som båda beror på två parametrar. Målet är att hitta par av parametrar så att båda matriserna är singulära. Denna avhandling är en sammanläggningsavhandling och består i huvudsak av 4 artiklar. Dessa artiklar berör både praktiska och teoretiska aspekter av de ovan nämnda beräkningsproblemen. I artikel A betraktas ett NEP som härstammar från en partiell differentialekvation, vilken beskriver vågutbredning i en vågledare. På det diskretiserade problemet tillämpas residual inversiteration (residual inverse iteration). Metoden kräver att man löser likartade linjära ekvationssystem många gånger, med olika högerled. När diskretiseringen blir noggrannare blir beräkningen av lösningen till det linjära ekvationssystemen en flaskhals. För att komma runt detta presenteras en Sylvester-baserad förkonditionerare som utnyttjar Sherman–Morrison–Woodburys formel för invertering av matriser med lågrangstermer. Artikel B behandlar den generaliserade Sylvesterekvationen och har två huvudresultat: Ett resultat är en karakterisering som under vissa antaganden motiverar existensen av lösningar som kan approximeras med matriser med låg rang. Resultatet är viktigt då många metoder för storskaliga problem har som mål att hitta en approximation av låg rang. Ett annat resultat är en Krylovmetod som kan användas när matriskoefficienterna lågrangskommuterar, d.v.s. när kommutatorn är en matris av låg rang. I artikel C undersöker vi den generaliserade Lyapunovekvationen. ALS-metoden, från engelskan alternating linear scheme, är en girig algoritm som presenterats i literaturen, och som iterativt utökar approximationen med en matris av rang 1. Denna utökning definieras utifrån att den är ett lokalt minimum av felet, när det senare mäts i en relaterad energinorm. Vi presenterar en utvidgning av den teoretiska motiveringen till användandet av ALS-metoden, från den stabila Lyapunovekvationen till den stabila generaliserade Lyapunovekvationen. Vi visar också på kopplingar till H2-optimal modellreduktion för bilinjära dynamiska system, och hur rang-1-uppdateringarna i ALS-metoden kan ses som lokalt H2-optimala till relaterade modellreduktionsproblem. Vi presenterar även varianter av den rationella Krylovmetoden som är anpassade till den generaliserade Lyapunovekvationen. Den fjärde artikeln, artikel D, presenterar en koppling mellan två-parameters egenvärdesproblemet och NEP. Genom att använda den ena ekvationen för att genomföra en variabeleliminering kan den andra skrivas som en familj av NEP:ar. Elimineringen sker på bekostnad av att ett generaliserat egenvärdesproblem behöver lösas för varje funktionsevaluering av NEP:en. Metoder för NEP kan på så sätt anpassas för att lösa två-parameters egenvärdesproblemet. Icke-linjärisering kan även tillämpas i omvänd riktning och kan på så sätt leda till linjäriseringar av vissa NEP:ar
Evaluation of a gypsum based fire seal by finite element calculations : Development of material parameters for Firesafe AS fire sealant, FS-GPG
No full textConstructing a workable computational model for a specific material can in many ways ease the design of the use of the material in different applications. More precise here, to be able to calculate the thickness of FS-GPG fire mortar sufficient for fire classification according to standards without being forced to make unnecessary tests.Designing a model with material parameters can be made in several different ways. It can either be done by practical inquiry, e.g. scanning different reactions, measure other parameters etcetera or you can make it by estimations of the properties based on the material compounds and construct a model by trial and error. FS-GPG resembles plasterboard/gypsum in many ways. Therefore the model is constructed with properties close to plasterboards properties found in literature. FS-GPG as well as plasterboard or gypsum contains considerable amounts of water. When the material is heated, free water will at first merge from the material. When further heated, dehydration will take place. Two reactions will occur at different temperatures due to dehydration where the gypsum forms new products. At these two reactions, energy is consumed by the water released from the crystals, delaying the temperature enhancement. The greatest achievement from the final model devloped is the comparison of calculated and measured temperature rise after 100 minutes in a specific application. They are almost identical.Validerat; 20141216 (global_studentproject_submitter
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
