Pavlos Peppas's research while affiliated with University of Patras and other places

We present an approach to incorporating qualitative assertions of conditional irrelevance into belief revision, in order to address the limitations of existing work which considers only unconditional irrelevance. These assertions serve to enforce the requirement of minimal change to existing beliefs, while also suggesting a route to reducing the computational cost of belief revision by excluding irrelevant beliefs from consideration. In our approach, a knowledge engineer specifies a collection of multivalued dependencies that encode domain-dependent assertions of conditional irrelevance in the knowledge base. We consider these as capturing properties of the underlying domain which should be taken into account during belief revision. We introduce two related notions of what it means for a multivalued dependency to be taken into account by a belief revision operator: partial and full compliance. We provide characterisations of partially and fully compliant belief revision operators in terms of semantic conditions on their associated faithful rankings. Using these characterisations, we show that the constraints for partially and fully compliant belief revision operators are compatible with the AGM postulates. Finally, we compare our approach to existing work on unconditional irrelevance in belief revision.

The prominent formal framework for belief change established by Alchourron, Gardenfors and Makinson circumscribes the territory of all rational belief-revision policies, encoded in the so-called AGM revision functions. A type of well-behaved and highly-expressive AGM revision function, induced from fixed total preorders over possible worlds, is that of uniform-revision operators (abbrev. UR operators). In this article, we introduce, both axiomatically and semantically (in terms of all popular constructive models for belief revision), a new class of AGM revision functions that is a proper sub-class of the class of AGM revision functions, but strictly larger (thus, more expressive) than the class of UR operators; hence, our proposal generalizes uniform revision. We call the presented operators theory-relational revision operators (abbrev. TR operators), since each such operator is uniquely defined through a single binary relation over theories of the language, called strong theory-relation. The connection between the semantic constructions of TR and UR operators is investigated, whereas, it is shown how a generalization (weakening) of a strong theory-relation ---which is also a single fixed binary relation over theories, called weak theory-relation--- can induce any AGM revision function. This latter result immediately proves an upper bound for the total number of AGM revision functions.

In this article, we identify some interesting types of rational revision operators that implement Horn revision. In particular, we first define (both axiomatically and semantically) a class of Horn revision operators based on proper set inclusion of the atoms satisfied by possible worlds. Furthermore, we show that a well-behaved type of rational revision, called uniform revision, is Horn-compliant. This is demonstrated by proving that concrete Horn revision operators implement particular uniform-revision policies.

In this article, we provide the epistemic-entrenchment and partial-meet characterizations of a new, important class of concrete revision operators (all of which satisfy the AGM postulates for revision), called Parametrized Difference revision operators (PD operators, for short). PD operators are natural generalizations of Dalal's revision operator, with a much greater range of applicability, hence, the epistemic-entrenchment and partial-meet characterizations of the latter are also provided, as a by-product. Lastly, we prove that PD operators satisfy the strong version of Parikh's relevance-sensitive axiom for belief revision, showing that they are fully compatible with the notion of relevance.

The AGM paradigm for belief change, as originally introduced by Alchourrón, Gärdenfors and Makinson, lacks any guidelines for the process of iterated revision. One of the most influential work addressing this problem is Darwiche and Pearl's approach (DP approach, for short), which, despite its well-documented shortcomings, remains to this date the most dominant. In this article, we make further observations on the DP approach. In particular, we prove that the DP postulates are, in a strong sense, inconsistent with Parikh's relevance-sensitive axiom (P), extending previous initial conflicts. Immediate consequences of this result are that an entire class of intuitive revision operators, which includes Dalal's operator, violates the DP postulates, as well as that the Independence postulate and Spohn's conditionalization are inconsistent with axiom (P). The whole study, essentially, indicates that two fundamental aspects of the revision process, namely, iteration and relevance, are in deep conflict, and opens the discussion for a potential reconciliation towards a comprehensive formal framework for knowledge dynamics.

The policy of rational belief revision is encoded in the so-called AGM revision functions. Such functions are characterized (both axiomatically and constructively) within the well-known AGM paradigm, proposed by Alchourron, Gardenfors and Makinson. In this article, we show that ---although not in a straightforward way--- a sufficient extension of the underlying language allows for the modelling of any AGM revision function (defined at the initial language), by means of a Hamming-based rule for belief revision introduced by Dalal (defined at the extended language). The established results enrich the applicability of Dalal's proposal, leading to a conceptual and ontological reduction, as well as open new doors for the construction of any type of revision function in a practical context, given the intuitive appeal and simplicity of Dalal's construction.

Parikh's relevance-sensitive axiom (P) for belief revision is open to two different interpretations, i.e., the weak and the strong version of (P), both of which are plausible depending on the context. Given that strong (P) has not received the attention it deserves, in this article, an extended examination of it is conducted. In particular, we point out interesting properties of the semantic characterization of the strong version of (P), as well as a vital feature of it that, potentially, results in a significant drop on the resources required for an implementation of a belief-revision system. Lastly, we shed light on the natural connection between global and local revision functions, via their corresponding semantic characterization, hence, a means for constructing global revision functions from local ones, and vice versa, is provided.

This paper summarizes the approaches employed by Team UTS Unleashed! to take First Place in the 2019 RoboCup@Home Social Standard Platform League. First, our system architecture is introduced. Next, our approach to basic skills needed for a strong performance in the competition. We describe several implementations for tests participation. Finally our software development methodology is discussed.

In this article, the epistemic-entrenchment and partial-meet characterizations of Parikh's relevance-sensitive axiom for belief revision, known as axiom (P), are provided. In short, axiom (P) states that, if a belief set K can be divided into two disjoint compartments, and the new information φ relates only to the first compartment, then the revision of K by φ should not affect the second compartment. Accordingly, we identify the subclass of epistemic-entrenchment and that of selection-function preorders, inducing AGM revision functions that satisfy axiom (P). Hence, together with the faithful-preorders characterization of (P) that has already been provided, Parikh's axiom is fully characterized in terms of all popular constructive models of Belief Revision. Since the notions of relevance and local change are inherent in almost all intellectual activity, the completion of the constructive view of (P) has a significant impact on many theoretical, as well as applied, domains of Artificial Intelligence.