Propositional dynamic logic of nonregular programs

In classical deterministic planning, solutions to planning tasks are simply sequences of actions, but that is not sufficient for contingent plans in non-deterministic environments. Contingent plans are often expressed through policies that map states to actions. An alternative is to specify contingent plans as programs, e.g. in the syntax of Propositional Dynamic Logic (PDL). PDL is a logic for reasoning about programs with sequential composition, test and non-deterministic choice. However, as we show in the paper, none of the existing PDL modalities directly captures the notion of a solution to a planning task under non-determinism. We add a new modality to star-free PDL correctly capturing this notion. We prove the appropriateness of the new modality by showing how to translate back and forth between policies and PDL programs under the new modality. More precisely, we show how a policy solution to a planning task gives rise to a program solution expressed via the new modality, and vice versa. We also provide an axiomatisation of our PDL extension through reduction axioms into standard star-free PDL.

Determinism and non-determinism in PDL

It is an open problem whether definability in Propositional Dynamic Logic (PDL) on forests is decidable. Based on an algebraic characterization by Boja\'nczyk, et. al.,(2012) in terms of forest algebras, Straubing (2013) described an approach to PDL based on a k-fold iterated distributive law. A proof that all languages satisfying such a k-fold iterated distributive law are in PDL would settle decidability of PDL. We solve this problem in the case k=2: All languages recognized by forest algebras satisfying a 2-fold iterated distributive law are in PDL. Furthermore, we show that this class is decidable. This provides a novel nontrivial decidable subclass of PDL, and demonstrates the viability of the proposed approach to deciding PDL in general.

Deterministic process logic is elementary

In its many guises and variations, propositional dynamic logic (PDL) plays an important role in various areas of computer science such as databases, artificial intelligence, and computer linguistics. One relevant and powerful variation is ICPDL, the extension of PDL with intersection and converse. Although ICPDL has several interesting applications, its computational properties have never been investigated. In this paper, we prove that ICPDL is decidable by developing a translation to the monadic second order logic of infinite trees. Our result has applications in information logic, description logic, and epistemic logic. In particular, we solve a long-standing open problem in information logic. Another virtue of our approach is that it provides a decidability proof that is more transparent than existing ones for PDL with intersection (but without converse).

The completeness proofs for Propositional Dynamic Logic (PDL) in the literature are non-constructive and usually presented in an informal manner. We obtain a formal and constructive completeness proof for Converse PDL by recasting a completeness proof by Kozen and Parikh into our constructive setting. We base our proof on a Pratt-style decision method for satisfiability constructing finite models for satisfiable formulas and pruning refutations for unsatisfiable formulas. Completeness of Segerberg's axiomatization of PDL is then obtained by translating pruning refutations to derivations in the Hilbert system. We first treat PDL without converse and then extend the proofs to Converse PDL. All results are formalized in Coq/Ssreflect.

We introduce dynamic algebras and show how they can be used to give an algebraic interpretation to propositional dynamic logic (PDL). Dynamic algebras include all Kripke models, the standard interpretation of PDL. We give a simple algebraic condition on *-free dynamic algebras that is necessary and sufficient for representation by *-free Kripke models. In the presence of*, the condition is sufficient for representation by a nonstandard Kripke model. This result leads to a duality between certain topological Kripke models and dynamic algebras analogous to the duality between Boolean algebras and their Stone spaces.

1 - Structures, Languages and Translations: the Structural Approach to Feature Logic

Agent-based systems have gained immense popularity in the last decade or so. Reasoning methods such as planning and decision-making under uncertainty applied to agents and multi-agent systems is an important area of research in agent-based systems. Most works in planning in uncertain environments incorporate sensing actions. We wish to address planning problems when there are no such sensing actions. For this, we introduce the notion of ‘try actions’ and develop a logic PDL_try that is an extension to propositional dynamic logic (PDL). The logic also allows specification of the implicit knowledge of an agent. We illustrate planning in our logic with a new adaptation of the popular blocks world domain. Example programs (plans) are discussed in detail for single agent and two agent situations.

Dynamic linear time temporal logic

Propositional dynamic logic with local assignment

Dynamic logic is a powerful framework for reasoning about imperative programs. An extension with a concurrent operator, called concurrent propositional dynamic logic (CPDL) [20], was introduced to formalise programs running in parallel. In a different direction, other authors proposed a systematic method for generating multi-valued propositional dynamic logics to reason about weighted programs [15]. This paper presents the first step of combining these two frameworks to introduce uncertainty in concurrent computations. In the proposed framework, a weight is assigned to each branch of the parallel execution, resulting in a (possible) asymmetric parallelism, inherent to the fuzzy programming paradigm [2, 23]. By adopting such an approach, a family of logics is obtained, called multi-valued concurrent propositional dynamic logics (\(\mathcal {GCDL}(\mathbf {A})\)), parametric on an action lattice \(\mathbf {A}\) specifying a notion of “weight” assigned to program execution. Additionally, the validity of some axioms of CPDL is discussed in the new family of generated logics.

Propositional Dynamic Logic can be extended to include both an infinitary iteration construct delta and a backtracking construct converse. The resulting logic does not satisfy the finite model property, but is nevertheless elementarily decidable. In order to establish this decidability result, deterministic two-way automata on infinite trees are defined and shown to be reducible to nondeterministic one-way automata on infinite trees. The decision procedure suggests a nonstandard semantics for the extended logic for which the finite model property does hold.

We investigate the relationship amongst some solutions to the frame problem. We encode Pednault’s syntax-based solution [20], Baker’s state-minimization policy [1], and Gelfond & Lifchitz’s Action Language A [7] in the propositional dynamic logic (PDL).T he formal relationships among these solutions are given. The results of the paper show that dynamic logic, as one of the formalisms for reasoning about dynamic domains, can be used as a formal tool for comparing and unifying logics of action.Keywordsrelationships between formalismsframe problemdynamic logic

Given the specification of an agent's behaviour in terms of what it can do at every task‐domain situation, we propose a mechanism to derive an equivalent specification but with fewer states that is based on Milner's ‘observation equivalence’. This is done by identifying internal states of an agent that are ‘observation equivalent’. We also introduce the notion of ‘pertinent fact’ about an agent's behaviour at an internal state as a fact that is true at all internal states of the agent that are ‘observation equivalent’ to this particular state, and also define a new logic, Propositional Logic of Knowledge and Actions (PLKA), to specify and reason about the behaviour of an agent in terms of pertinent facts. A complete axiomatization of this logic, which stands as a conservative extension of both the logic S5 of knowledge and Propositional Dynamic Logic (PDL), and a characterization of an agent's behaviour in terms of ‘pertinent facts’ are also provided.