Language-Emptiness Checking of Alternating Tree Automata Using Symbolic Reachability Analysis

AbstractAlternating tree automata and AND/OR graphs provide elegant formalisms that enable branching- time logics to be verified in linear time. The seminal work of Kupferman et al. [Orna Kupferman, Moshe Y. Vardi, and Pierre Wolper. An automata-theoretic approach to branching-time model checking. J. ACM, 47(2):312–360, 2000] showed that 1) branching-time model checking is reducible to the language non-emptiness checking of the product of two alternating automata representing the model and property under verification, and 2) the non-emptiness problem can be solved by performing a search on an AND/OR graph representing this product. Their algorithm, however, can only be implemented in an explicit-state model checker because it needs stacks to detect accept and reject runs. In this paper, we propose a BDD-based approach to check the language non-emptiness of the product automaton. We use a technique called “state recording” from Schuppan and Biere [Viktor Schuppan and Armin Biere. Efficient reduction of finite state model checking to reachability analysis. Int. Journal on Software Tools for Technology Transfer (STTT), 5(2–3):185–204, 2004] to emulate the stack mechanism from explicit-state model checking. This technique allows us to transform the product automaton into a well-defined AND/OR graph. We develop a BDD-based reachability algorithm to efficiently determine whether a solution graph for the AND/OR graph exists and thereby solve the model-checking problem. While “state recording” increases the size of the state space, the advantage of our approach lies in the memory saving BDDs can offer and the potential it opens up for optimisation of the reachability analysis. We remark that this technique always detects the shortest counter-example

Formal symbolic verification using heuristic search and abstraction techniques

Computing devices are pervading our everyday life and imposing challenges for designersthat have the responsibility of producing reliable hardware and software systems. As systemsgrow in size and complexity, it becomes increasingly difficult to verify whether a design works asintended. Conventional verification methods, such as simulation and testing, exercise only partsof the system and from these parts, draw conclusions about the correctness of the total design.For complex designs, the parts of the system that can be verified are relatively small. Formalverification aims to overcome this problem. Instead of exercising the system, formal verificationbuilds mathematical models of designs and proves whether properties hold in these models. Indoing so, it at least aims to cover the complete design. Model checking is a formal verificationmethod that automatically verifies a model of a design, or generates diagnostic information ifthe model cannot be verified. It is because of this usability and level of automation that modelchecking has gained a high degree of success in verifying circuit designs.The major disadvantage of model checking is its poor scalability. This is due to its algorithmicnature: namely, every state of the model needs to be enumerated. In practice, properties ofinterest may not need the exhaustive enumeration of the model state space. Many propertiescan be verified (or falsified) by examining a small number of states. In such cases, exhaustivealgorithms can be replaced with search algorithms that are directed by heuristics. Methods basedon heuristics generally scale well.This thesis investigates non-exhaustive model checking algorithms and focuses on error detectionin system verification. The approach is based on a novel integration of symbolic model checking,heuristic search and abstraction techniques to produce a framework that we call abstractiondirectedmodel checking. There are 3 main components in this framework. First, binary decisiondiagrams (BDDs) and heuristic search are combined to develop a symbolic heuristic search algorithm.This algorithm is used to detect errors. Second, abstraction techniques are applied inan iterative way. In the initial phase, the model is abstracted, and this model is verified usingexhaustive algorithms. If a definitive verification result cannot be obtained, the same abstractionis re-used to generate a search heuristic. The heuristic in turn is used to direct a searchalgorithm that searches for error states in the concrete model. Third, a model transformationmechanism converts an arbitrary branching-time property to a reachability property. Essentially,this component allows this framework to be applied to a more general class of temporal property.By amalgamating these three components, the framework offers a new verification methodologythat speeds up error detection in formal verification. The current implementation of this frameworkindicates that it can outperform existing standard techniques both in run-time and memoryconsumption, and scales much better than conventional model checking

Optimized dispatch of wind farms with power control capability for power system restoration

Abstract As the power control technology of wind farms develops, the output power of wind farms can be constant, which makes it possible for wind farms to participate in power system restoration. However, due to the uncertainty of wind energy, the actual output power can’t reach a constant dispatch power in all time intervals, resulting in uncertain power sags which may induce the frequency of the system being restored to go outside the security limits. Therefore, it is necessary to optimize the dispatch of wind farms participating in power system restoration. Considering that the probability distribution function (PDF) of transient power sags is hard to obtain, a robust optimization model is proposed in this paper, which can maximize the output power of wind farms participating in power system restoration. Simulation results demonstrate that the security constraints of the restored system can be kept within security limits when wind farm dispatch is optimized by the proposed method

In vitro study of the antioxidative and antiproliferative capabilities of Lactobacillus casei 16-fermented soymilk

In this study, soymilk was fermented with Lactobacillus casei 16. The contents of aglycone isoflavones, saponins, total phenolic, and free amino acid in the fermented soymilk, and the protection for the HepG2 cells against 2,2'-azobis(2-amidinopropane) dihydrochloride (ABAP) damage and the antiproliferative effects of the fermented soymilk on the HT-29 cells and Caco-2 cells were studied. The results showed that the levels of total phenolic, aglycone isoflavones, and free amino acids in the L. casei 16-fermented soymilk were enhanced. The ethanol extract and the water extract of the L. casei 16-fermented soymilk showed protection for HepG2 cells against ABAP damage and inhibited the proliferation of the HT-29 cells and Caco-2 cells, which may be attributed to the enhanced level of total phenolic, aglycone isoflavones, and free amino acids in the L. casei 16-fermented soymilk