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This paper shows equivalence of applicative similarity and contextual approximation, and hence also of bisimilarity and contextual equivalence, in LR, the deterministic call-by-need lambda calculus with letrec extended by data constructors, case-expressions and Haskell's seqoperator. LR models an untyped version of the core language of Haskell. Bisimilarity simplifies equivalence proofs in the calculus and opens a way for more convenient correctness proofs for program transformations.
The proof is by a fully abstract and surjective transfer of the contextual approximation into a call-by-name calculus, which is an extension of Abramsky's lazy lambda calculus. In the latter calculus equivalence of similarity and contextual approximation can be shown by Howe's method. Using an equivalent but inductive definition of behavioral preorder we then transfer similarity back to the calculus LR.
The translation from the call-by-need letrec calculus into the extended call-by-name lambda calculus is the composition of two translations. The first translation replaces the call-by-need strategy by a call-by-name strategy and its correctness is shown by exploiting infinite tress, which emerge by unfolding the letrec expressions. The second translation encodes letrec-expressions by using multi-fixpoint combinators and its correctness is shown syntactically by comparing reductions of both calculi. A further result of this paper is an isomorphism between the mentioned calculi, and also with a call-by-need letrec calculus with a less complex definition of reduction than LR.
Poster presentation: Calcium plays a pivotal role in relaying electrical signals of the cell to subcellular compartments, such as the nucleus. Since this one ion type is used by the cell for many processes a neuron needs to establish finely tuned calcium pathways in order to be able to differentiate multiple tasks, [1-3].
While it is known that neurons can actively change their shape upon neuronal activity, [4-7], we here present novel findings of activity-regulated nuclear morphology, [8,9]. With the help of an experimental and computational modeling approach, we show that hippocampal neurons can change the previously spherical shape of their nuclei to complex and infolded morphologies. This morphology regulation is demonstrated to be regulated by NMDA-receptor gated calcium, while synaptic and extra-synaptic NMDA-receptors elicit opposing effects on nuclear morphology, [8].
The structural alterations of the cell nucleus have significant effects on nuclear calcium dynamics. Compartmentalization of the nucleus, due to membrane infoldings, changes calcium frequencies, amplitudes and spatial distributions, [8,10]. Since these parameters have been shown to control downstream events towards gene transcription, [11,12], the results elucidate the cellular control of nuclear function with the help of morphology modulation. With respect to processes downstream of calcium, we show that histone H3 phosphorylation is closely linked to nuclear morphology. Investigating the nuclear morphologies of hippocampal neurons, two major classes were identified [9,10]. One class contains non-infolded nuclei that have the function of calcium signal integrators, while the other class contains highly infolded nuclei, which function as frequency detectors of nuclear calcium, [10].
Extending this interdisciplinary approach of investigating structure/function relationships in neurons, the effects of cellular morphology – as well as the morphology of the endoplasmic reticulum and other organelles – on neuronal calcium signals is currently being investigated. This endeavor makes use of highly detailed, three-dimensional models of neuronal calcium dynamics, including the three-dimensional morphology of the cell and its organelles.
Conceptual design of an ALICE Tier-2 centre integrated into a multi-purpose computing facility
(2012)
This thesis discusses the issues and challenges associated with the design and operation of a data analysis facility for a high-energy physics experiment at a multi-purpose computing centre. At the spotlight is a Tier-2 centre of the distributed computing model of the ALICE experiment at the Large Hadron Collider at CERN in Geneva, Switzerland. The design steps, examined in the thesis, include analysis and optimization of the I/O access patterns of the user workload, integration of the storage resources, and development of the techniques for effective system administration and operation of the facility in a shared computing environment. A number of I/O access performance issues on multiple levels of the I/O subsystem, introduced by utilization of hard disks for data storage, have been addressed by the means of exhaustive benchmarking and thorough analysis of the I/O of the user applications in the ALICE software framework. Defining the set of requirements to the storage system, describing the potential performance bottlenecks and single points of failure and examining possible ways to avoid them allows one to develop guidelines for selecting the way how to integrate the storage resources. The solution, how to preserve a specific software stack for the experiment in a shared environment, is presented along with its effects on the user workload performance. The proposal for a flexible model to deploy and operate the ALICE Tier-2 infrastructure and applications in a virtual environment through adoption of the cloud computing technology and the 'Infrastructure as Code' concept completes the thesis. Scientific software applications can be efficiently computed in a virtual environment, and there is an urgent need to adapt the infrastructure for effective usage of cloud resources.
This thesis will first introduce in more detail the Bayesian theory and its use in integrating multiple information sources. I will briefly talk about models and their relation to the dynamics of an environment, and how to combine multiple alternative models. Following that I will discuss the experimental findings on multisensory integration in humans and animals. I start with psychophysical results on various forms of tasks and setups, that show that the brain uses and combines information from multiple cues. Specifically, the discussion will focus on the finding that humans integrate this information in a way that is close to the theoretical optimal performance. Special emphasis will be put on results about the developmental aspects of cue integration, highlighting experiments that could show that children do not perform similar to the Bayesian predictions. This section also includes a short summary of experiments on how subjects handle multiple alternative environmental dynamics. I will also talk about neurobiological findings of cells receiving input from multiple receptors both in dedicated brain areas but also primary sensory areas. I will proceed with an overview of existing theories and computational models of multisensory integration. This will be followed by a discussion on reinforcement learning (RL). First I will talk about the original theory including the two different main approaches model-free and model-based reinforcement learning. The important variables will be introduced as well as different algorithmic implementations. Secondly, a short review on the mapping of those theories onto brain and behaviour will be given. I mention the most in uential papers that showed correlations between the activity in certain brain regions with RL variables, most prominently between dopaminergic neurons and temporal difference errors. I will try to motivate, why I think that this theory can help to explain the development of near-optimal cue integration in humans. The next main chapter will introduce our model that learns to solve the task of audio-visual orienting. Many of the results in this section have been published in [Weisswange et al. 2009b,Weisswange et al. 2011]. The model agent starts without any knowledge of the environment and acts based on predictions of rewards, which will be adapted according to the reward signaling the quality of the performed action. I will show that after training this model performs similarly to the prediction of a Bayesian observer. The model can also deal with more complex environments in which it has to deal with multiple possible underlying generating models (perform causal inference). In these experiments I use di#erent formulations of Bayesian observers for comparison with our model, and find that it is most similar to the fully optimal observer doing model averaging. Additional experiments using various alterations to the environment show the ability of the model to react to changes in the input statistics without explicitly representing probability distributions. I will close the chapter with a discussion on the benefits and shortcomings of the model. The thesis continues whith a report on an application of the learning algorithm introduced before to two real world cue integration tasks on a robotic head. For these tasks our system outperforms a commonly used approximation to Bayesian inference, reliability weighted averaging. The approximation is handy because of its computational simplicity, because it relies on certain assumptions that are usually controlled for in a laboratory setting, but these are often not true for real world data. This chapter is based on the paper [Karaoguz et al. 2011]. Our second modeling approach tries to address the neuronal substrates of the learning process for cue integration. I again use a reward based training scheme, but this time implemented as a modulation of synaptic plasticity mechanisms in a recurrent network of binary threshold neurons. I start the chapter with an additional introduction section to discuss recurrent networks and especially the various forms of neuronal plasticity that I will use in the model. The performance on a task similar to that of chapter 3 will be presented together with an analysis of the in uence of different plasticity mechanisms on it. Again benefits and shortcomings and the general potential of the method will be discussed. I will close the thesis with a general conclusion and some ideas about possible future work.
The Symposium on Theoretical Aspects of Computer Science (STACS) is held alternately in France and in Germany. The conference of February 26-28, 2009, held in Freiburg, is the 26th in this series. Previous meetings took place in Paris (1984), Saarbr¨ucken (1985), Orsay (1986), Passau (1987), Bordeaux (1988), Paderborn (1989), Rouen (1990), Hamburg (1991), Cachan (1992), W¨urzburg (1993), Caen (1994), M¨unchen (1995), Grenoble (1996), L¨ubeck (1997), Paris (1998), Trier (1999), Lille (2000), Dresden (2001), Antibes (2002), Berlin (2003), Montpellier (2004), Stuttgart (2005), Marseille (2006), Aachen (2007), and Bordeaux (2008). ...
From 12.12.2010 to 17.12.2010, the Dagstuhl Seminar 10501 "Advances and Applications of Automata on Words and Trees" was held in Schloss Dagstuhl - Leibniz Center for Informatics. During the seminar, several participants presented their current research, and ongoing work and open problems were discussed. Abstracts of the presentations given during the seminar as well as abstracts of seminar results and ideas are put together in this paper. The first section describes the seminar topics and goals in general. Links to extended abstracts or full papers are provided, if available.
Seminar: 10501 - Advances and Applications of Automata on Words and Trees. The aim of the seminar was to discuss and systematize the recent fast progress in automata theory and to identify important directions for future research. For this, the seminar brought together more than 40 researchers from automata theory and related fields of applications. We had 19 talks of 30 minutes and 5 one-hour lectures leaving ample room for discussions. In the following we describe the topics in more detail.
This article shows that there exist two particular linear orders such that first-order logic with these two linear orders has the same expressive power as first-order logic with the Bit-predicate FO(Bit). As a corollary we obtain that there also exists a built-in permutation such that first-order logic with a linear order and this permutation is as expressive as FO(Bit).
The calculus CHF models Concurrent Haskell extended by concurrent, implicit futures. It is a process calculus with concurrent threads, monadic concurrent evaluation, and includes a pure functional lambda-calculus which comprises data constructors, case-expressions, letrec-expressions, and Haskell’s seq. Futures can be implemented in Concurrent Haskell using the primitive unsafeInterleaveIO, which is available in most implementations of Haskell. Our main result is conservativity of CHF, that is, all equivalences of pure functional expressions are also valid in CHF. This implies that compiler optimizations and transformations from pure Haskell remain valid in Concurrent Haskell even if it is extended by futures. We also show that this is no longer valid if Concurrent Haskell is extended by the arbitrary use of unsafeInterleaveIO.
We show how Sestoft’s abstract machine for lazy evaluation of purely functional programs can be extended to evaluate expressions of the calculus CHF – a process calculus that models Concurrent Haskell extended by imperative and implicit futures. The abstract machine is modularly constructed by first adding monadic IO-actions to the machine and then in a second step we add concurrency. Our main result is that the abstract machine coincides with the original operational semantics of CHF, w.r.t. may- and should-convergence.
This paper considers the logic FOcard, i.e., first-order logic with cardinality predicates that can specify the size of a structure modulo some number. We study the expressive power of FOcard on the class of languages of ranked, finite, labelled trees with successor relations. Our first main result characterises the class of FOcard-definable tree languages in terms of algebraic closure properties of the tree languages. As it can be effectively checked whether the language of a given tree automaton satisfies these closure properties, we obtain a decidable characterisation of the class of regular tree languages definable in FOcard. Our second main result considers first-order logic with unary relations, successor relations, and two additional designated symbols < and + that must be interpreted as a linear order and its associated addition. Such a formula is called addition-invariant if, for each fixed interpretation of the unary relations and successor relations, its result is independent of the particular interpretation of < and +. We show that the FOcard-definable tree languages are exactly the regular tree languages definable in addition-invariant first-order logic. Our proof techniques involve tools from algebraic automata theory, reasoning with locality arguments, and the use of logical interpretations. We combine and extend methods developed by Benedikt and Segoufin (ACM ToCL, 2009) and Schweikardt and Segoufin (LICS, 2010).
Diese Arbeit untersucht den Einfluss des Game-Design auf ausgelöste Lernprozesse und den Erfolg von Serious Games. Hierzu werden Game-Design Paradigmen entwickelt, die als Richtlinien für Konzeption und Umsetzung eines Serious Game dienen. Als Serious Games werden Videospiele bezeichnet, die zur Wissensvermittlung konzipiert worden sind. Dabei sollen die motivationalen Faktoren eines Videospiels genutzt werden, um einen intrinsisch motivierten Lernprozess auszulösen. Das Bewertungkriterium für den Erfolg einer Spielmechanik ist somit die Erfüllung der Lernziele. Damit dieses Erfolgskriterium genauer untersucht werden kann, werden die ausgelösten Lernprozesse differenziert betrachtet. In der Literatur werden folgende Lernprozesse hervorgehoben: Der Prozess des Erfahrungslernens und metakognitive Prozesse. Darüber hinaus sind Eigenschaften der Zielgruppe, wie Alter oder Geschlecht weitere wichtige Faktoren. Das dieser Arbeit zu Grunde liegende Forschungsframework setzt sich wie folgt zusammen: Lernszenario, Lernprozess und Lernerfolg. Das Lernszenario ist durch folgende Faktoren charakterisiert: Game Characteristics (Eigenschaften des Serious Game), Instructional Content (Arbeitsanweisungen und Trainingsetting) sowie Player Characteristics (Eigenschaften der Zielgruppe). Diese Parameter bedingen den Lernprozess, welcher unter dem Aspekt des Erfahrungslernens und der Metakognition analysiert wird. Eine besondere Problemstellung in den Player Characteristics ergibt sich aus dem sogenannten Net-Generation Konflikt. Mit Net-Generation wird die Generation bezeichnet, welche mit neuen Medien wie Internet und mobiler Kommunikation aufgewachsen ist. Diese besitzt im Unterschied zu älteren Generationen ein anderes Lernverhalten. Um die Aspekte des Net-Generation Konflikts und die Auswirkungen auf den Lernprozesses untersuchen zu können, wird ein Serious Game entwickelt, dessen Spielmechanik sich an folgenden Game-Design Paradigmen ausrichtet: Akzeptanz, Leichte Zugänglichkeit, Spielspaß und die Unterstützung des Lernprozesses. Dieses Serious Game FISS (Fertigungs- und Instandhaltungs-Strategie Simulation) wird bei der Daimler AG seit 2008 zur Ausbildung von Ingenieuren eingesetzt. FISS simuliert eine Fertigungslinie, die mit Hilfe geeigneter Wartungsstrategien und effizientem Personaleinsatz erfolgreich geführt werden soll. Die Spielmechanik orientiert sich an dem Genre der Rundenstrategie und wird in einem Anwesenheitstraining im Team durchgeführt. Hervorzuheben ist, dass die Zielgruppe bezüglich des Alters inhomogen ist und deshalb der Net-Generation Konflikt berücksichtigt werden muss. Im Anschluss wird FISS unter folgenden Aspekten untersucht: Der Prozess des Erfahrungslernens, metakognitive Prozesse und die Integration der Non-Net-Generation. Die Ergebnisse zeigen, dass die Eigenschaften des Game-Design einen signifikanten Einfluss auf den Prozess des Erfahrungslernens und die Lernerfolge besitzen. Spieler mit einem praktischen Zugang zu Lerninhalten (Concrete Experience) erzielten einen signifikant größeren Wissenzuwachs. Zudem profitierten alle Spieler von FISS, jedoch konnte in einer Vorstudie kein Einfluss metakognitiver Fähigkeiten auf den Wissenzuwachs nachgewiesen werden. Die weitere zentrale Studie dieser Arbeit fokussiert den Net-Generation Konflikt und evaluiert den Erfolg der eingangs aufgestellten Game-Design Paradigmen. Hierzu werden die Teilnehmer nach drei Altersgruppen getrennt betrachtet: Non-Net-Generation, Net-Generation und die dazwischen liegende Crossover-Generation. Es zeigt sich, dass der Lern- und Spielerfolg aller Generationen gleichermaßen signifikant ist und nur innerhalb des zu erwartenden Standardfehlers abweicht. FISS eignet sich folglich für alle Generationen. Diese Ergebnisse können stellvertretend für Serious Games im Genre der Rundenstrategie gesehen werden. Die in dieser Arbeit erzielten Ergebnisse ermöglichen ein besseres Verständnis der Auswirkungen des Game-Design auf den Lernerfolg. Hiermit können potentielle Schwachstellen eines Serious Game erkannt und vermieden werden. Die Erkenntnisse im Bereich des Erfahrungslernens ermöglichen zudem eine bessere Anpassungen an die Zielgruppe. Für die zukünftige Forschung wurde mit dem in dieser Arbeit entwickelten Framework eine Grundlage geschaffen.
This thesis combines behavioral and cognitive approaches regarding the Web for analyzing users' behavior and supposed interests.
The work is placed in a new field of research called Web Science, which includes, but is not restricted to, the analysis of the World Wide Web. The term Web Science is affected by Tim Berners-Lee et al., who invited the researchers to "create a science of the web" [BLHH+06a]. The thesis is structured in two parts, reflecting the intersection of disciplines that is required for Web Science.
The first part is related to computer science and information systems. This part defines the Gugubarra concepts and algorithms for web user profiling and builds upon the results by Mushtaq et al. [MWTZ04]. This profiling aims at understanding the behavior and supposed interests of users. Based on these concepts, a framework was implemented to support the needs of web site owners. The core technologies used are Java, Spring, Hibernate, and content management systems. The design principles, architecture, implementation, and tests of the prototype are reported.
The second part is directly related to behavioral economics and is connected to the areas of economics, mathematics, and psychology. This part contributes to behavior models, as was claimed by Tim Berners-Lee et al.: "Though individual users may or may not be rational, it has long been noted that en masse people behave as utility maximisers. In that case, understanding the incentives that are available to web users should provide methods for generating models of behaviour..."[BLHH+06b]. The focus here is on studies that investigate the user's choice of online information services in a multi-attribute context. The introduced research framework takes into account background and local context effects and builds upon theoretical foundations by Tversky and Kahneman [TK86]. The findings provide useful insights to behavioral scientists and to practitioners on how to use framing strategies to alter the user's choice.
Succinctness is a natural measure for comparing the strength of different logics. Intuitively, a logic L_1 is more succinct than another logic L_2 if all properties that can be expressed in L_2 can be expressed in L_1 by formulas of (approximately) the same size, but some properties can be expressed in L_1 by (significantly) smaller formulas.
We study the succinctness of logics on linear orders. Our first theorem is concerned with the finite variable fragments of first-order logic. We prove that:
(i) Up to a polynomial factor, the 2- and the 3-variable fragments of first-order logic on linear orders have the same succinctness. (ii) The 4-variable fragment is exponentially more succinct than the 3-variable fragment. Our second main result compares the succinctness of first-order logic on linear orders with that of monadic second-order logic. We prove that the fragment of monadic second-order logic that has the same expressiveness as first-order logic on linear orders is non-elementarily more succinct than first-order logic.
A generalization of the compressed string pattern match that applies to terms with variables is investigated: Given terms s and t compressed by singleton tree grammars, the task is to find an instance of s that occurs as a subterm in t. We show that this problem is in NP and that the task can be performed in time O(ncjVar(s)j), including the construction of the compressed substitution, and a representation of all occurrences. We show that the special case where s is uncompressed can be performed in polynomial time. As a nice application we show that for an equational deduction of t to t0 by an equality axiom l = r (a rewrite) a single step can be performed in polynomial time in the size of compression of t and l; r if the number of variables is fixed in l. We also show that n rewriting steps can be performed in polynomial time, if the equational axioms are compressed and assumed to be constant for the rewriting sequence. Another potential application are querying mechanisms on compressed XML-data bases.
Correctness of program transformations in extended lambda calculi with a contextual semantics is usually based on reasoning about the operational semantics which is a rewrite semantics. A successful approach to proving correctness is the combination of a context lemma with the computation of overlaps between program transformations and the reduction rules.The method is similar to the computation of critical pairs for the completion of term rewriting systems. We describe an effective unification algorithm to determine all overlaps of transformations with reduction rules for the lambda calculus LR which comprises a recursive let-expressions, constructor applications, case expressions and a seq construct for strict evaluation. The unification algorithm employs many-sorted terms, the equational theory of left-commutativity modeling multi-sets, context variables of different kinds and a mechanism for compactly representing binding chains in recursive let-expressions. As a result the algorithm computes a finite set of overlappings for the reduction rules of the calculus LR that serve as a starting point to the automatization of the analysis of program transformations.
In this paper we analyze the semantics of a higher-order functional language with concurrent threads, monadic IO and synchronizing variables as in Concurrent Haskell. To assure declarativeness of concurrent programming we extend the language by implicit, monadic, and concurrent futures. As semantic model we introduce and analyze the process calculus CHF, which represents a typed core language of Concurrent Haskell extended by concurrent futures. Evaluation in CHF is defined by a small-step reduction relation. Using contextual equivalence based on may- and should-convergence as program equivalence, we show that various transformations preserve program equivalence. We establish a context lemma easing those correctness proofs. An important result is that call-by-need and call-by-name evaluation are equivalent in CHF, since they induce the same program equivalence. Finally we show that the monad laws hold in CHF under mild restrictions on Haskell’s seq-operator, which for instance justifies the use of the do-notation.
The well-known proof of termination of reduction in simply typed calculi is adapted to a monomorphically typed lambda-calculus with case and constructors and recursive data types. The proof differs at several places from the standard proof. Perhaps it is useful and can be extended also to more complex calculi.