@inproceedings{3853, abstract = {Quantitative languages are an extension of boolean languages that assign to each word a real number. Mean-payoff automata are finite automata with numerical weights on transitions that assign to each infinite path the long-run average of the transition weights. When the mode of branching of the automaton is deterministic, nondeterministic, or alternating, the corresponding class of quantitative languages is not robust as it is not closed under the pointwise operations of max, min, sum, and numerical complement. Nondeterministic and alternating mean-payoff automata are not decidable either, as the quantitative generalization of the problems of universality and language inclusion is undecidable. We introduce a new class of quantitative languages, defined by mean-payoff automaton expressions, which is robust and decidable: it is closed under the four pointwise operations, and we show that all decision problems are decidable for this class. Mean-payoff automaton expressions subsume deterministic meanpayoff automata, and we show that they have expressive power incomparable to nondeterministic and alternating mean-payoff automata. We also present for the first time an algorithm to compute distance between two quantitative languages, and in our case the quantitative languages are given as mean-payoff automaton expressions.}, author = {Chatterjee, Krishnendu and Doyen, Laurent and Edelsbrunner, Herbert and Henzinger, Thomas A and Rannou, Philippe}, location = {Paris, France}, pages = {269 -- 283}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum für Informatik}, title = {{Mean-payoff automaton expressions}}, doi = {10.1007/978-3-642-15375-4_19}, volume = {6269}, year = {2010}, } @inproceedings{3854, abstract = {Graph games of infinite length provide a natural model for open reactive systems: one player (Eve) represents the controller and the other player (Adam) represents the environment. The evolution of the system depends on the decisions of both players. The specification for the system is usually given as an ω-regular language L over paths and Eve’s goal is to ensure that the play belongs to L irrespective of Adam’s behaviour. The classical notion of winning strategies fails to capture several interesting scenarios. For example, strong fairness (Streett) conditions are specified by a number of request-grant pairs and require every pair that is requested infinitely often to be granted infinitely often: Eve might win just by preventing Adam from making any new request, but a “better” strategy would allow Adam to make as many requests as possible and still ensure fairness. To address such questions, we introduce the notion of obliging games, where Eve has to ensure a strong condition Φ, while always allowing Adam to satisfy a weak condition Ψ. We present a linear time reduction of obliging games with two Muller conditions Φ and Ψ to classical Muller games. We consider obliging Streett games and show they are co-NP complete, and show a natural quantitative optimisation problem for obliging Streett games is in FNP. We also show how obliging games can provide new and interesting semantics for multi-player games.}, author = {Chatterjee, Krishnendu and Horn, Florian and Löding, Christof}, location = {Paris, France}, pages = {284 -- 296}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum für Informatik}, title = {{Obliging games}}, doi = {10.1007/978-3-642-15375-4_20}, volume = {6269}, year = {2010}, } @inproceedings{3855, abstract = {We study observation-based strategies for partially-observable Markov decision processes (POMDPs) with parity objectives. An observation-based strategy relies on partial information about the history of a play, namely, on the past sequence of observations. We consider qualitative analysis problems: given a POMDP with a parity objective, decide whether there exists an observation-based strategy to achieve the objective with probability 1 (almost-sure winning), or with positive probability (positive winning). Our main results are twofold. First, we present a complete picture of the computational complexity of the qualitative analysis problem for POMDPs with parity objectives and its subclasses: safety, reachability, Büchi, and coBüchi objectives. We establish several upper and lower bounds that were not known in the literature. Second, we give optimal bounds (matching upper and lower bounds) for the memory required by pure and randomized observation-based strategies for each class of objectives.}, author = {Chatterjee, Krishnendu and Doyen, Laurent and Henzinger, Thomas A}, location = {Brno, Czech Republic}, pages = {258 -- 269}, publisher = {Springer}, title = {{Qualitative analysis of partially-observable Markov Decision Processes}}, doi = {10.1007/978-3-642-15155-2_24}, volume = {6281}, year = {2010}, } @inproceedings{3856, abstract = {We consider two-player zero-sum games on graphs. These games can be classified on the basis of the information of the players and on the mode of interaction between them. On the basis of information the classification is as follows: (a) partial-observation (both players have partial view of the game); (b) one-sided complete-observation (one player has complete observation); and (c) complete-observation (both players have complete view of the game). On the basis of mode of interaction we have the following classification: (a) concurrent (players interact simultaneously); and (b) turn-based (players interact in turn). The two sources of randomness in these games are randomness in transition function and randomness in strategies. In general, randomized strategies are more powerful than deterministic strategies, and randomness in transitions gives more general classes of games. We present a complete characterization for the classes of games where randomness is not helpful in: (a) the transition function (probabilistic transition can be simulated by deterministic transition); and (b) strategies (pure strategies are as powerful as randomized strategies). As consequence of our characterization we obtain new undecidability results for these games. }, author = {Chatterjee, Krishnendu and Doyen, Laurent and Gimbert, Hugo and Henzinger, Thomas A}, location = {Brno, Czech Republic}, pages = {246 -- 257}, publisher = {Springer}, title = {{Randomness for free}}, doi = {10.1007/978-3-642-15155-2_23}, volume = {6281}, year = {2010}, } @inproceedings{3857, abstract = {We consider probabilistic automata on infinite words with acceptance defined by safety, reachability, Büchi, coBüchi, and limit-average conditions. We consider quantitative and qualitative decision problems. We present extensions and adaptations of proofs for probabilistic finite automata and present an almost complete characterization of the decidability and undecidability frontier of the quantitative and qualitative decision problems for probabilistic automata on infinite words.}, author = {Chatterjee, Krishnendu and Henzinger, Thomas A}, location = {Singapore, Singapore}, pages = {1 -- 16}, publisher = {Springer}, title = {{Probabilistic Automata on infinite words: decidability and undecidability results}}, doi = {10.1007/978-3-642-15643-4_1}, volume = {6252}, year = {2010}, } @inproceedings{3858, abstract = {We consider two-player zero-sum games on graphs. On the basis of the information available to the players these games can be classified as follows: (a) partial-observation (both players have partial view of the game); (b) one-sided partial-observation (one player has partial-observation and the other player has complete-observation); and (c) complete-observation (both players have com- plete view of the game). We survey the complexity results for the problem of de- ciding the winner in various classes of partial-observation games with ω-regular winning conditions specified as parity objectives. We present a reduction from the class of parity objectives that depend on sequence of states of the game to the sub-class of parity objectives that only depend on the sequence of observations. We also establish that partial-observation acyclic games are PSPACE-complete.}, author = {Chatterjee, Krishnendu and Doyen, Laurent}, location = {Yogyakarta, Indonesia}, pages = {1 -- 14}, publisher = {Springer}, title = {{The complexity of partial-observation parity games}}, doi = {10.1007/978-3-642-16242-8_1}, volume = {6397}, year = {2010}, } @proceedings{3859, abstract = {This book constitutes the proceedings of the 8th International Conference on Formal Modeling and Analysis of Timed Systems, FORMATS 2010, held in Klosterneuburg, Austria in September 2010. The 14 papers presented were carefully reviewed and selected from 31 submissions. In addition, the volume contains 3 invited talks and 2 invited tutorials.The aim of FORMATS is to promote the study of fundamental and practical aspects of timed systems, and to bring together researchers from different disciplines that share an interest in the modeling and analysis of timed systems. Typical topics include foundations and semantics, methods and tools, and applications.}, editor = {Chatterjee, Krishnendu and Henzinger, Thomas A}, location = {Klosterneuburg, Austria}, publisher = {Springer}, title = {{Formal modeling and analysis of timed systems}}, doi = {10.1007/978-3-642-15297-9}, volume = {6246}, year = {2010}, } @inproceedings{3860, abstract = {In mean-payoff games, the objective of the protagonist is to ensure that the limit average of an infinite sequence of numeric weights is nonnegative. In energy games, the objective is to ensure that the running sum of weights is always nonnegative. Generalized mean-payoff and energy games replace individual weights by tuples, and the limit average (resp. running sum) of each coordinate must be (resp. remain) nonnegative. These games have applications in the synthesis of resource-bounded processes with multiple resources. We prove the finite-memory determinacy of generalized energy games and show the inter- reducibility of generalized mean-payoff and energy games for finite-memory strategies. We also improve the computational complexity for solving both classes of games with finite-memory strategies: while the previously best known upper bound was EXPSPACE, and no lower bound was known, we give an optimal coNP-complete bound. For memoryless strategies, we show that the problem of deciding the existence of a winning strategy for the protagonist is NP-complete.}, author = {Chatterjee, Krishnendu and Doyen, Laurent and Henzinger, Thomas A and Raskin, Jean}, location = {Chennai, India}, pages = {505 -- 516}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum für Informatik}, title = {{Generalized mean-payoff and energy games}}, doi = {10.4230/LIPIcs.FSTTCS.2010.505}, volume = {8}, year = {2010}, } @article{3861, abstract = {We introduce strategy logic, a logic that treats strategies in two-player games as explicit first-order objects. The explicit treatment of strategies allows us to specify properties of nonzero-sum games in a simple and natural way. We show that the one-alternation fragment of strategy logic is strong enough to express the existence of Nash equilibria and secure equilibria, and subsumes other logics that were introduced to reason about games, such as ATL, ATL*, and game logic. We show that strategy logic is decidable, by constructing tree automata that recognize sets of strategies. While for the general logic, our decision procedure is nonelementary, for the simple fragment that is used above we show that the complexity is polynomial in the size of the game graph and optimal in the size of the formula (ranging from polynomial to 2EXPTIME depending on the form of the formula).}, author = {Chatterjee, Krishnendu and Henzinger, Thomas A and Piterman, Nir}, journal = {Information and Computation}, number = {6}, pages = {677 -- 693}, publisher = {Elsevier}, title = {{Strategy logic}}, doi = {10.1016/j.ic.2009.07.004}, volume = {208}, year = {2010}, } @article{3863, abstract = {We consider two-player parity games with imperfect information in which strategies rely on observations that provide imperfect information about the history of a play. To solve such games, i.e., to determine the winning regions of players and corresponding winning strategies, one can use the subset construction to build an equivalent perfect-information game. Recently, an algorithm that avoids the inefficient subset construction has been proposed. The algorithm performs a fixed-point computation in a lattice of antichains, thus maintaining a succinct representation of state sets. However, this representation does not allow to recover winning strategies. In this paper, we build on the antichain approach to develop an algorithm for constructing the winning strategies in parity games of imperfect information. One major obstacle in adapting the classical procedure is that the complementation of attractor sets would break the invariant of downward-closedness on which the antichain representation relies. We overcome this difficulty by decomposing problem instances recursively into games with a combination of reachability, safety, and simpler parity conditions. We also report on an experimental implementation of our algorithm: to our knowledge, this is the first implementation of a procedure for solving imperfect-information parity games on graphs.}, author = {Berwanger, Dietmar and Chatterjee, Krishnendu and De Wulf, Martin and Doyen, Laurent and Henzinger, Thomas A}, journal = {Information and Computation}, number = {10}, pages = {1206 -- 1220}, publisher = {Elsevier}, title = {{Strategy construction for parity games with imperfect information}}, doi = {10.1016/j.ic.2009.09.006}, volume = {208}, year = {2010}, } @inproceedings{3864, abstract = {Often one has a preference order among the different systems that satisfy a given specification. Under a probabilistic assumption about the possible inputs, such a preference order is naturally expressed by a weighted automaton, which assigns to each word a value, such that a system is preferred if it generates a higher expected value. We solve the following optimal-synthesis problem: given an omega-regular specification, a Markov chain that describes the distribution of inputs, and a weighted automaton that measures how well a system satisfies the given specification tinder the given input assumption, synthesize a system that optimizes the measured value. For safety specifications and measures that are defined by mean-payoff automata, the optimal-synthesis problem amounts to finding a strategy in a Markov decision process (MDP) that is optimal for a long-run average reward objective, which can be done in polynomial time. For general omega-regular specifications, the solution rests on a new, polynomial-time algorithm for computing optimal strategies in MDPs with mean-payoff parity objectives. We present some experimental results showing optimal systems that were automatically generated in this way.}, author = {Chatterjee, Krishnendu and Henzinger, Thomas A and Jobstmann, Barbara and Singh, Rohit}, location = {Edinburgh, United Kingdom}, pages = {380 -- 395}, publisher = {Springer}, title = {{Measuring and synthesizing systems in probabilistic environments}}, doi = {10.1007/978-3-642-14295-6_34}, volume = {6174}, year = {2010}, } @inproceedings{3865, abstract = {We introduce a technique for debugging multi-threaded C programs and analyzing the impact of source code changes, and its implementation in the prototype tool DIRECT. Our approach uses a combination of source code instrumentation and runtime management. The source code along with a test harness is instrumented to monitor Operating System (OS) and user defined function calls. DIRECT tracks all concurrency control primitives and, optionally, data from the program. DIRECT maintains an abstract global state that combines information from every thread, including the sequence of function calls and concurrency primitives executed. The runtime manager can insert delays, provoking thread inter-leavings that may exhibit bugs that are difficult to reach otherwise. The runtime manager collects an approximation of the reachable state space and uses this approximation to assess the impact of change in a new version of the program.}, author = {Chatterjee, Krishnendu and De Alfaro, Luca and Raman, Vishwanath and Sánchez, César}, editor = {Rosenblum, David and Taenzer, Gabriele}, location = {Paphos, Cyprus}, pages = {293 -- 307}, publisher = {Springer}, title = {{Analyzing the impact of change in multi-threaded programs}}, doi = {10.1007/978-3-642-12029-9_21}, volume = {6013}, year = {2010}, } @inproceedings{3866, abstract = {Systems ought to behave reasonably even in circumstances that are not anticipated in their specifications. We propose a definition of robustness for liveness specifications which prescribes, for any number of environment assumptions that are violated, a minimal number of system guarantees that must still be fulfilled. This notion of robustness can be formulated and realized using a Generalized Reactivity formula. We present an algorithm for synthesizing robust systems from such formulas. For the important special case of Generalized Reactivity formulas of rank 1, our algorithm improves the complexity of [PPS06] for large specifications with a small number of assumptions and guarantees.}, author = {Bloem, Roderick and Chatterjee, Krishnendu and Greimel, Karin and Henzinger, Thomas A and Jobstmann, Barbara}, editor = {Touili, Tayssir and Cook, Byron and Jackson, Paul}, location = {Edinburgh, UK}, pages = {410 -- 424}, publisher = {Springer}, title = {{Robustness in the presence of liveness}}, doi = {10.1007/978-3-642-14295-6_36}, volume = {6174}, year = {2010}, } @article{3867, abstract = {Weighted automata are nondeterministic automata with numerical weights on transitions. They can define quantitative languages L that assign to each word w a real number L(w). In the case of infinite words, the value of a run is naturally computed as the maximum, limsup, liminf, limit-average, or discounted-sum of the transition weights. The value of a word w is the supremum of the values of the runs over w. We study expressiveness and closure questions about these quantitative languages. We first show that the set of words with value greater than a threshold can be omega-regular for deterministic limit-average and discounted-sum automata, while this set is always omega-regular when the threshold is isolated (i.e., some neighborhood around the threshold contains no word). In the latter case, we prove that the omega-regular language is robust against small perturbations of the transition weights. We next consider automata with transition weights 0 or 1 and show that they are as expressive as general weighted automata in the limit-average case, but not in the discounted-sum case. Third, for quantitative languages L-1 and L-2, we consider the operations max(L-1, L-2), min(L-1, L-2), and 1 - L-1, which generalize the boolean operations on languages, as well as the sum L-1 + L-2. We establish the closure properties of all classes of quantitative languages with respect to these four operations.}, author = {Chatterjee, Krishnendu and Doyen, Laurent and Henzinger, Thomas A}, journal = {Logical Methods in Computer Science}, number = {3}, pages = {1 -- 23}, publisher = {International Federation of Computational Logic}, title = {{Expressiveness and closure properties for quantitative languages}}, doi = {10.2168/LMCS-6(3:10)2010}, volume = {6}, year = {2010}, } @article{3868, abstract = {Simulation and bisimulation metrics for stochastic systems provide a quantitative generalization of the classical simulation and bisimulation relations. These metrics capture the similarity of states with respect to quantitative specifications written in the quantitative mu-calculus and related probabilistic logics. We first show that the metrics provide a bound for the difference in long-run average and discounted average behavior across states, indicating that the metrics can be used both in system verification, and in performance evaluation. For turn-based games and MDPs, we provide a polynomial-time algorithm for the computation of the one-step metric distance between states. The algorithm is based on linear programming; it improves on the previous known exponential-time algorithm based on a reduction to the theory of reals. We then present PSPACE algorithms for both the decision problem and the problem of approximating the metric distance between two states, matching the best known algorithms for Markov chains. For the bisimulation kernel of the metric our algorithm works in time O(n(4)) for both turn-based games and MDPs; improving the previously best known O(n(9).log(n)) time algorithm for MDPs. For a concurrent game G, we show that computing the exact distance be tween states is at least as hard as computing the value of concurrent reachability games and the square-root-sum problem in computational geometry. We show that checking whether the metric distance is bounded by a rational r, can be done via a reduction to the theory of real closed fields, involving a formula with three quantifier alternations, yielding O(vertical bar G vertical bar(O(vertical bar G vertical bar 5))) time complexity, improving the previously known reduction, which yielded O(vertical bar G vertical bar(O(vertical bar G vertical bar 7))) time complexity. These algorithms can be iterated to approximate the metrics using binary search}, author = {Chatterjee, Krishnendu and De Alfaro, Luca and Majumdar, Ritankar and Raman, Vishwanath}, journal = {Logical Methods in Computer Science}, number = {3}, pages = {1 -- 27}, publisher = {International Federation of Computational Logic}, title = {{Algorithms for game metrics}}, doi = {10.2168/LMCS-6(3:13)2010}, volume = {6}, year = {2010}, } @article{3901, abstract = {We are interested in 3-dimensional images given as arrays of voxels with intensity values. Extending these values to acontinuous function, we study the robustness of homology classes in its level and interlevel sets, that is, the amount of perturbationneeded to destroy these classes. The structure of the homology classes and their robustness, over all level and interlevel sets, can bevisualized by a triangular diagram of dots obtained by computing the extended persistence of the function. We give a fast hierarchicalalgorithm using the dual complexes of oct-tree approximations of the function. In addition, we show that for balanced oct-trees, thedual complexes are geometrically realized in $R^3$ and can thus be used to construct level and interlevel sets. We apply these tools tostudy 3-dimensional images of plant root systems.}, author = {Bendich, Paul and Edelsbrunner, Herbert and Kerber, Michael}, journal = {IEEE Transactions of Visualization and Computer Graphics}, number = {6}, pages = {1251 -- 1260}, publisher = {IEEE}, title = {{Computing robustness and persistence for images}}, doi = {10.1109/TVCG.2010.139}, volume = {16}, year = {2010}, } @phdthesis{3962, author = {Pflicke, Holger}, issn = {2663-337X}, publisher = {Institute of Science and Technology Austria}, title = {{Dendritic cell migration across basement membranes in the skin}}, year = {2010}, } @article{4134, abstract = {All species are restricted in their distribution. Currently, ecological models can only explain such limits if patches vary in quality, leading to asymmetrical dispersal, or if genetic variation is too low at the margins for adaptation. However, population genetic models suggest that the increase in genetic variance resulting from dispersal should allow adaptation to almost any ecological gradient. Clearly therefore, these models miss something that prevents evolution in natural populations. We developed an individual-based simulation to explore stochastic effects in these models. At high carrying capacities, our simulations largely agree with deterministic predictions. However, when carrying capacity is low, the population fails to establish for a wide range of parameter values where adaptation was expected from previous models. Stochastic or transient effects appear critical around the boundaries in parameter space between simulation behaviours. Dispersal, gradient steepness, and population density emerge as key factors determining adaptation on an ecological gradient. }, author = {Bridle, Jon and Polechova, Jitka and Kawata, Masakado and Butlin, Roger}, journal = {Ecology Letters}, number = {4}, pages = {485 -- 494}, publisher = {Wiley-Blackwell}, title = {{Why is adaptation prevented at ecological margins? New insights from individual-based simulations}}, doi = {10.1111/j.1461-0248.2010.01442.x}, volume = {13}, year = {2010}, } @article{4157, abstract = {Integrin- and cadherin-mediated adhesion is central for cell and tissue morphogenesis, allowing cells and tissues to change shape without loosing integrity. Studies predominantly in cell culture showed that mechanosensation through adhesion structures is achieved by force-mediated modulation of their molecular composition. The specific molecular composition of adhesion sites in turn determines their signalling activity and dynamic reorganization. Here, we will review how adhesion sites respond to mecanical stimuli, and how spatially and temporally regulated signalling from different adhesion sites controls cell migration and tissue morphogenesis.}, author = {Papusheva, Ekaterina and Heisenberg, Carl-Philipp J}, journal = {EMBO Journal}, number = {16}, pages = {2753 -- 2768}, publisher = {Wiley-Blackwell}, title = {{Spatial organization of adhesion: force-dependent regulation and function in tissue morphogenesis}}, doi = {10.1038/emboj.2010.182}, volume = {29}, year = {2010}, } @article{4243, abstract = {We investigate a new model for populations evolving in a spatial continuum. This model can be thought of as a spatial version of the Lambda-Fleming-Viot process. It explicitly incorporates both small scale reproduction events and large scale extinction-recolonisation events. The lineages ancestral to a sample from a population evolving according to this model can be described in terms of a spatial version of the Lambda-coalescent. Using a technique of Evans (1997), we prove existence and uniqueness in law for the model. We then investigate the asymptotic behaviour of the genealogy of a finite number of individuals sampled uniformly at random (or more generally `far enough apart') from a two-dimensional torus of sidelength L as L tends to infinity. Under appropriate conditions (and on a suitable timescale) we can obtain as limiting genealogical processes a Kingman coalescent, a more general Lambda-coalescent or a system of coalescing Brownian motions (with a non-local coalescence mechanism).}, author = {Barton, Nicholas H and Etheridge, Alison and Véber, Amandine}, journal = {Electronic Journal of Probability}, number = {7}, pages = {162 -- 216}, publisher = {Institute of Mathematical Statistics}, title = {{A new model for evolution in a spatial continuum}}, doi = {10.1214/EJP.v15-741}, volume = {15}, year = {2010}, }