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Search Results: 1 - 10 of 170 matches for " Tommi Junttila "
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Simulating Parity Reasoning (extended version)
Tero Laitinen,Tommi Junttila,Ilkka Niemel?
Computer Science , 2013,
Abstract: Propositional satisfiability (SAT) solvers, which typically operate using conjunctive normal form (CNF), have been successfully applied in many domains. However, in some application areas such as circuit verification, bounded model checking, and logical cryptanalysis, instances can have many parity (xor) constraints which may not be handled efficiently if translated to CNF. Thus, extensions to the CNF-driven search with various parity reasoning engines ranging from equivalence reasoning to incremental Gaussian elimination have been proposed. This paper studies how stronger parity reasoning techniques in the DPLL(XOR) framework can be simulated by simpler systems: resolution, unit propagation, and parity explanations. Such simulations are interesting, for example, for developing the next generation SAT solvers capable of handling parity constraints efficiently.
Extending Clause Learning SAT Solvers with Complete Parity Reasoning (extended version)
Tero Laitinen,Tommi Junttila,Ilkka Niemel?
Computer Science , 2012,
Abstract: Instances of logical cryptanalysis, circuit verification, and bounded model checking can often be succinctly represented as a combined satisfiability (SAT) problem where an instance is a combination of traditional clauses and parity constraints. This paper studies how such combined problems can be efficiently solved by augmenting a modern SAT solver with an xor-reasoning module in the DPLL(XOR) framework. A new xor-reasoning module that deduces all possible implied literals using incremental Gauss-Jordan elimination is presented. A decomposition technique that can greatly reduce the size of parity constraint matrices while allowing still to deduce all implied literals is presented. It is shown how to eliminate variables occuring only in parity constraints while preserving the decomposition. The proposed techniques are evaluated experimentally.
SMT-based Induction Methods for Timed Systems
Roland Kindermann,Tommi Junttila,Ilkka Niemel?
Computer Science , 2012,
Abstract: Modeling time related aspects is important in many applications of verification methods. For precise results, it is necessary to interpret time as a dense domain, e.g. using timed automata as a formalism, even though the system's resulting infinite state space is challenging for verification methods. Furthermore, fully symbolic treatment of both timing related and non-timing related elements of the state space seems to offer an attractive approach to model checking timed systems with a large amount of non-determinism. This paper presents an SMT-based timed system extension to the IC3 algorithm, a SAT-based novel, highly efficient, complete verification method for untimed systems. Handling of the infinite state spaces of timed system in the extended IC3 algorithm is based on suitably adapting the well-known region abstraction for timed systems. Additionally, $k$-induction, another symbolic verification method for discrete time systems, is extended in a similar fashion to support timed systems. Both new methods are evaluated and experimentally compared to a booleanization-based verification approach that uses the original discrete time IC3 algorithm.
Conflict-Driven XOR-Clause Learning (extended version)
Tero Laitinen,Tommi Junttila,Ilkka Niemel?
Computer Science , 2014,
Abstract: Modern conflict-driven clause learning (CDCL) SAT solvers are very good in solving conjunctive normal form (CNF) formulas. However, some application problems involve lots of parity (xor) constraints which are not necessarily efficiently handled if translated into CNF. This paper studies solving CNF formulas augmented with xor-clauses in the DPLL(XOR) framework where a CDCL SAT solver is coupled with a separate xor-reasoning module. New techniques for analyzing xor-reasoning derivations are developed, allowing one to obtain smaller CNF clausal explanations for xor-implied literals and also to derive and learn new xor-clauses. It is proven that these new techniques allow very short unsatisfiability proofs for some formulas whose CNF translations do not have polynomial size resolution proofs, even when a very simple xor-reasoning module capable only of unit propagation is applied. The efficiency of the proposed techniques is evaluated on a set of challenging logical cryptanalysis instances.
Bounded Model Checking of an MITL Fragment for Timed Automata
Roland Kindermann,Tommi Junttila,Ilkka Niemel?
Computer Science , 2013,
Abstract: Timed automata (TAs) are a common formalism for modeling timed systems. Bounded model checking (BMC) is a verification method that searches for runs violating a property using a SAT or SMT solver. MITL is a real-time extension of the linear time logic LTL. Originally, MITL was defined for traces of non-overlapping time intervals rather than the "super-dense" time traces allowing for intervals overlapping in single points that are employed by the nowadays common semantics of timed automata. In this paper we extend the semantics of a fragment of MITL to super-dense time traces and devise a bounded model checking encoding for the fragment. We prove correctness and completeness in the sense that using a sufficiently large bound a counter-example to any given non-holding property can be found. We have implemented the proposed bounded model checking approach and experimentally studied the efficiency and scalability of the implementation.
Classifying and Propagating Parity Constraints (extended version)
Tero Laitinen,Tommi Junttila,Ilkka Niemel?
Computer Science , 2014,
Abstract: Parity constraints, common in application domains such as circuit verification, bounded model checking, and logical cryptanalysis, are not necessarily most efficiently solved if translated into conjunctive normal form. Thus, specialized parity reasoning techniques have been developed in the past for propagating parity constraints. This paper studies the questions of deciding whether unit propagation or equivalence reasoning is enough to achieve full propagation in a given parity constraint set. Efficient approximating tests for answering these questions are developed. It is also shown that equivalence reasoning can be simulated by unit propagation by adding a polynomial amount of redundant parity constraints to the problem. It is proven that without using additional variables, an exponential number of new parity constraints would be needed in the worst case. The presented classification and propagation methods are evaluated experimentally.
Linear Encodings of Bounded LTL Model Checking
Armin Biere,Keijo Heljanko,Tommi Junttila,Timo Latvala,Viktor Schuppan
Computer Science , 2006, DOI: 10.2168/LMCS-2(5:5)2006
Abstract: We consider the problem of bounded model checking (BMC) for linear temporal logic (LTL). We present several efficient encodings that have size linear in the bound. Furthermore, we show how the encodings can be extended to LTL with past operators (PLTL). The generalised encoding is still of linear size, but cannot detect minimal length counterexamples. By using the virtual unrolling technique minimal length counterexamples can be captured, however, the size of the encoding is quadratic in the specification. We also extend virtual unrolling to Buchi automata, enabling them to accept minimal length counterexamples. Our BMC encodings can be made incremental in order to benefit from incremental SAT technology. With fairly small modifications the incremental encoding can be further enhanced with a termination check, allowing us to prove properties with BMC. Experiments clearly show that our new encodings improve performance of BMC considerably, particularly in the case of the incremental encoding, and that they are very competitive for finding bugs. An analysis of the liveness-to-safety transformation reveals many similarities to the BMC encodings in this paper. Using the liveness-to-safety translation with BDD-based invariant checking results in an efficient method to find shortest counterexamples that complements the BMC-based approach.
Characterization of a transcriptome from a non-model organism, Cladonia rangiferina, the grey reindeer lichen, using high-throughput next generation sequencing and EST sequence data
Junttila Sini,Rudd Stephen
BMC Genomics , 2012, DOI: 10.1186/1471-2164-13-575
Abstract: Background Lichens are symbiotic organisms that have a remarkable ability to survive in some of the most extreme terrestrial climates on earth. Lichens can endure frequent desiccation and wetting cycles and are able to survive in a dehydrated molecular dormant state for decades at a time. Genetic resources have been established in lichen species for the study of molecular systematics and their taxonomic classification. No lichen species have been characterised yet using genomics and the molecular mechanisms underlying the lichen symbiosis and the fundamentals of desiccation tolerance remain undescribed. We report the characterisation of a transcriptome of the grey reindeer lichen, Cladonia rangiferina, using high-throughput next-generation transcriptome sequencing and traditional Sanger EST sequencing data. Results Altogether 243,729 high quality sequence reads were de novo assembled into 16,204 contigs and 49,587 singletons. The genome of origin for the sequences produced was predicted using Eclat with sequences derived from the axenically grown symbiotic partners used as training sequences for the classification model. 62.8% of the sequences were classified as being of fungal origin while the remaining 37.2% were predicted as being of algal origin. The assembled sequences were annotated by BLASTX comparison against a non-redundant protein sequence database with 34.4% of the sequences having a BLAST match. 29.3% of the sequences had a Gene Ontology term match and 27.9% of the sequences had a domain or structural match following an InterPro search. 60 KEGG pathways with more than 10 associated sequences were identified. Conclusions Our results present a first transcriptome sequencing and de novo assembly for a lichen species and describe the ongoing molecular processes and the most active pathways in C. rangiferina. This brings a meaningful contribution to publicly available lichen sequence information. These data provide a first glimpse into the molecular nature of the lichen symbiosis and characterise the transcriptional space of this remarkable organism. These data will also enable further studies aimed at deciphering the genetic mechanisms behind lichen desiccation tolerance.
Green, Innovative, and Profitable: A Case Study of Managerial Capabilities at Interface Inc.
Tommi Lampikoski
Technology Innovation Management Review , 2012,
Abstract: This article describes the pioneering green-innovation management practices of a resource-intensive corporation, Interface Inc., which is a globally operating carpet manufacturer. Even during the current economic downturn, many companies remain committed to advancing their green business agendas. However, recent research suggests that most of these companies are far from reaching substantial competitive advantage from this commitment because they lack the connection between their green agendas and core innovation-management activities. This study illustrates how Interface succeeded with radical green innovations by investing in managerial capabilities that allowed it to conduct research, recognize opportunities, and revolutionize the carpeting industry. These capabilities enabled Interface to continuously challenge and disrupt well-established management recipes, existing knowledge, and proven industrial practices, and they enabled it to create a sustainable competitive advantage through a winning portfolio of radical green innovations.
Arukihenro: Reflections on Fieldwork by a Visual Anthropologist
MENDEL, Tommi
Electronic Journal of Contemporary Japanese Studies , 2008,
Abstract:
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