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Petri nets are one of the most popular tools for modeling distributed systems. This book provides a modern look at the theory behind them, by studying three classes of nets that model (i) sequential systems, (ii) non-communicating parallel systems, and (iii) communicating parallel systems. A decidable and causality respecting behavioral equivalence is presented for each class, followed by a modal logic characterization for each equivalence. The author then introduces a suitable process algebra for the corresponding class of nets and proves that the behavioral equivalence proposed for each class is a congruence for the operator of the corresponding process algebra. Finally, an axiomatization of the behavioral congruence is proposed. The theory is introduced step by step, with ordinary-language explanations and examples provided throughout, to remain accessible to readers without specialized training in concurrency theory or formal logic. Exercises with solutions solidify understanding, and the final chapter hints at extensions of the theory.
The chapter examines the motivational dWPHP problem from three perspectives: logical (axiomatization and provability), computational complexity (witnessing) and proof complexity (propositional translation). It also defines strong proof systems and formulates some of their properties.
The chapter considers (variants of) the Nisan-Wigderson generator as a proof complexity generator, formulates Razborov's conjecture about it and examines some proof complexity limitations of such generators.
The chapter gives the historic background in bounded arithmetic and describes how it lead to the development of the presented theory. It lists prerequisites and some notation and terminology to be used.
The chapter introduces the gadget generator, shows its hardness for some specific proof systems and examines its disjunction hardness. It proves (modulo a computational hypothesis) the hardness.
The chapter defines the notion of a generator and its hardness, and formulates the hardness conjecture. It also defines a stronger notion of pseudosurjectivity of a generator and states the key conjecture about it. It examines some consequences of the two conjectures for the dWPHP problem. It also relates the hardness conjecture to feasible interpolation, gives a model-theoretic view of the issues and discusses a relation to pseudorandomness.
The chapter gives several consistency results related to the dWPHP problem. It also considers the hardness conjecture for feasibly infinite NP sets. It relates witnessing of dWPHP to various computational complexity conjectures.
This final chapter offers a number of topics for further research involving ordinary PHP, S-T computations, a new notion of PLS-infinite NP sets, proof search algorithms, an exponential time weakening of generators and the function inversion problem.
The chapter concentrates on the pivotal case of extended resolution. It recalls some characterizations of its lengths-of-proofs function and formulates a framework for lower bounds proofs using expansions of pseudofinite structures. It gives an example of a specific candidate construction.
The chapter presents several topics outside of the theory where some ideas and results around proof complexity generators appear to be relevant. These include SAT algorithms, the optimality problem of proof complexity, structured PHP approach, the incompleteness theorem and total NP search problems.
The chapter examines possible stretch of generators and relates it to problems about Kolmogorov's complexity, the feasible disjunction property and to a related notion of disjunction hardness of generators. The truth-table function is presented as a key example of a generator and its hardness and pseudosurjectivity is considered. A problem about time-bounded Kolmogorov complexity is formulated.