Indistinguishability Relation Research Articles

Logical equivalences, homomorphism indistinguishability, and forbidden minors

Two graphs G and H are homomorphism indistinguishable over a graph class F if for all graphs F∈F the number of homomorphisms from F to G is equal to the number of homomorphisms from F to H. Many graph isomorphism relaxations such as (quantum) isomorphism, spectral, and logical equivalences can be characterised as homomorphism indistinguishability relations over certain graph classes.Abstracting from the wealth of such instances, we show in this paper that equivalences w.r.t. any self-complementarity logic admitting a characterisation as homomorphism indistinguishability relation can be characterised by homomorphism indistinguishability over a minor-closed graph class. Self-complementarity is a mild property satisfied by most well-studied logics.Furthermore, we classify all graph classes which are in a sense finite and satisfy the maximality condition of being homomorphism distinguishing closed, i.e. adding any graph to the class strictly refines its homomorphism indistinguishability relation. Thereby, we answer various questions raised by Roberson (2022) on properties of the homomorphism distinguishing closure.

Lasserre Hierarchy for Graph Isomorphism and Homomorphism Indistinguishability

We show that feasibility of the $t^\text{th}$ level of the Lasserre semidefinite programming hierarchy for graph isomorphism can be expressed as a homomorphism indistinguishability relation. In other words, we define a class $\mathcal{L}_t$ of graphs such that graphs $G$ and $H$ are not distinguished by the $t^\text{th}$ level of the Lasserre hierarchy if and only if they admit the same number of homomorphisms from any graph in $\mathcal{L}_t$. By analysing the treewidth of graphs in $\mathcal{L}_t$, we prove that the $3t^\text{th}$ level of Sherali--Adams linear programming hierarchy is as strong as the $t^\text{th}$ level of Lasserre. Moreover, we show that this is best possible in the sense that $3t$ cannot be lowered to $3t-1$ for any $t$. The same result holds for the Lasserre hierarchy with non-negativity constraints, which we similarly characterise in terms of homomorphism indistinguishability over a family $\mathcal{L}_t^+$ of graphs. Additionally, we give characterisations of level-$t$ Lasserre with non-negativity constraints in terms of logical equivalence and via a graph colouring algorithm akin to the Weisfeiler--Leman algorithm. This provides a polynomial time algorithm for determining if two given graphs are distinguished by the $t^\text{th}$ level of the Lasserre hierarchy with non-negativity constraints.

Uncertainty-based knowing how logic

Abstract We introduce a novel semantics for a multi-agent epistemic operator of knowing how, based on an indistinguishability relation between plans. Our proposal is, arguably, closer to the standard presentation of knowing that modalities in classical epistemic logic. We study the relationship between this new semantics and previous approaches, showing that our setting is general enough to capture them. We also study the logical properties of the new semantics. First, we define a sound and complete axiomatization. Second, we define a suitable notion of bisimulation and prove correspondence theorems. Finally, we investigate the computational complexity of the model checking and satisfiability problems for the new logic.

Uncertainty-Based Semantics for Multi-Agent Knowing How Logics

We introduce a new semantics for a multi-agent epistemic operator of knowing how, based on an indistinguishability relation between plans. Our proposal is, arguably, closer to the standard presentation of knowing that modalities in classical epistemic logic. We study the relationship between this semantics and previous approaches, showing that our setting is general enough to capture them. We also define a sound and complete axiomatization, and investigate the computational complexity of its model checking and satisfiability problems.

Simplicial complexes and closure systems induced by indistinguishability relations

In this paper, we develop in a more general mathematical context the notion of indistinguishability, which in graph theory has recently been investigated as a symmetry relation with respect to a fixed vertex subset. The starting point of our analysis is to consider a set Ω of functions defined on a universe set U and to define an equivalence relation ≡A on U for any subset A⊆Ω in the following way: u≡Au′ if a(u)=a(u′) for any function a∈A. By means of this family of relations, we introduce the indistinguishability relation ≈ on the power set P(Ω) as follows: for A,A′∈P(Ω), we set A≈A′ if the relations ≡A and ≡A′ coincide. We use then the indistinguishability relation ≈ to introduce several set families on Ω that have interesting order, matroidal and combinatorial properties. We call the above set families the indistinguishability structures of the function system(U,Ω). Furthermore, we obtain a closure system and an abstract simplicial complex interacting each other by means of three hypergraphs having relevance in both theoretical computer science and graph theory. The first part of this paper is devoted to investigate the basic mathematical properties of the indistinguishability structures for arbitrary function systems. The second part deals with some specific cases of study derived from simple undirected graphs and the usual Euclidean real line.

Micro and macro models of granular computing induced by the indiscernibility relation

In rough set theory (RST), and more generally in granular computing on information tables (GRC-IT), a central tool is the Pawlak’s indiscernibility relation between objects of a universe set with respect to a fixed attribute subset. Let us observe that Pawlak’s relation induces in a natural way an equivalence relation ≈ on the attribute power set that identifies two attribute subsets yielding the same indiscernibility partition. We call indistinguishability relation of a given information table I the equivalence relation ≈, that can be considered as a kind of global indiscernibility. In this paper we investigate the mathematical foundations of indistinguishability relation through the introduction of two new structures that are, respectively, a complete lattice and an abstract simplicial complex. We show that these structures can be studied at both a micro granular and a macro granular level and that are naturally related to the core and the reducts of I. We first discuss the role of these structures in GrC-IT by providing some interpretations, then we prove several mathematical results concerning the fundamental properties of such structures.

Analysis of behaviour of automata

This survey contains results concerning behavioural and abstract theory of finite automata. Finite automata and related constructions are cornerstone concepts of discrete mathematics and mathematical cybernetics. The automata theory finds multiple applications in programming, technical cybernetics, theory of dynamic systems, etc. Classical problems of this theory are the direct problems: analysis of processes of transformation of information carried out by automata and properties of automata, and the inverse problems: synthesis of automata of prescribed properties and identification (restoring, recognition, deciphering, control and diagnosis) of an automaton by means of an experiment with it. The fundamental notion which underlies the study of the above problems is the notion of a fragment which is a natural extension of a great body of known fragments of particular forms possessing their characteristic properties. We introduce the fragment–automaton relation, consider properties of classes of fragments of a given automaton. We introduce the notion of a cofragment (forbidden fragment) of an automaton, consider properties of classes of automata which have a given fragment–cofragment pair. Another key notion is the notion of an identifier of unobservable components of functioning of an automaton, that is, of a fragment which allows for unambiguous determination of values of these components. It is demonstrated that identifiers provide us with a powerful tool for solving the problems we consider in this paper. Next, we introduce the general notion of representation of the reference automaton to within a given precision (similarity) relative to a priori class of automata as a pair (fragment, cofragment) of the reference automaton which can be a pair (fragment, cofragment) of an automaton in a priori class if and only if it is similar to the reference one. It is shown that this concept covers and generalises a series of particular notions (control, recognising experiments, questionnaire languages, k -sets, etc.) known in the automata theory. We suggest a natural classification of representations. We carry out a systematic study of conditions for existence and structure of representations. We obtain precise conditions for existence of representations of general form and their particular classes in terms of relations between properties of a priori class, class of automata similar to the reference one, and the class of automata indistinguishable from the reference automaton for the corresponding indistinguishability relation. We obtain conditions for existence of nontrivial representations for various classes, including questionnaire languages and control experiments. This survey contains a series of final results, but they can be treated as a regular step in studying problems of analysis and synthesis of automata by their behaviour, which are continuously filled with new content and require additional resources to solve them.

INTRANSITIVITY AND VAGUENESS

There are many examples in the literature that suggest that indistinguishability is intransitive, despite the fact that the indistinguishability relation is typically taken to be an equivalence relation (and thus transitive). It is shown that if the uncertainty perception and the question of when an agent reports that two things are indistinguishable are both carefully modeled, the problems disappear, and indistinguishability can indeed be taken to be an equivalence relation. Moreover, this model also suggests a logic of vagueness that seems to solve many of the problems related to vagueness discussed in the philosophical literature. In particular, it is shown here how the logic can handle the sorites paradox.

Notions of Sameness by Default and their Application to Anaphora, Vagueness, and Uncertain Reasoning

We motivate and formalize the idea of sameness by default: two objects are considered the same if they cannot be proved to be different. This idea turns out to be useful for a number of widely different applications, including natural language processing, reasoning with incomplete information, and even philosophical paradoxes. We consider two formalizations of this notion, both of which are based on Reiter's Default Logic. The first formalization is a new relation of indistinguishability that is introduced by default. We prove that the corresponding default theory has a unique extension, in which every two objects are indistinguishable if and only if their non-equality cannot be proved from the known facts. We show that the indistinguishability relation has some desirable properties: it is reflexive, symmetric, and, while not transitive, it has a transitive "flavor." The second formalization is an extension (modification) of the ordinary language equality by a similar default: two objects are equal if and only if their non-equality cannot be proved from the known facts. It appears to be less elegant from a formal point of view. In particular, it gives rise to multiple extensions. However, this extended equality is better suited for most of the applications discussed in this paper.

A reformulation of entropy in the presence of indistinguishability operators

This paper deals with the measurement of entropy when an indistinguishability relation on the set of events has been defined. Our approach states that entropy could be measured in terms of the observed distinguishability of the set of events. In this sense, the “observer paradigm” is introduced, and definitions for joint and conditional entropy under this paradigm are given. We also present some interesting properties and relationships with Shannon's entropy measure.

Modular correctness proofs of behavioural implementations

We introduce a concept of behavioural implementation for algebraic specifications which is based on an indistinguishability relation (called behavioural equality). The central objective of this work is the investigation of proof rules which allow us to establish the correctness of behavioural implementations in a modular (and stepwise) way and, moreover, are practicable enough to induce proof obligations that can be discharged with existing theorem provers. Under certain conditions our proof technique can also be applied for proving the correctness of implementations based on an abstraction equivalence between algebras in the sense of Sannella and Tarlecki. The whole approach is presented in the framework of total algebras and first-order logic with equality.

On behavioural abstraction and behavioural satisfaction in higher-order logic

The behavioural semantics of specifications with higher-order logical formulae as axioms is analyzed. A characterization of behavioural abstraction via behavioural satisfaction of formulae in which the equality symbol is interpreted as indistinguishability, which is due to Reichel and was recently generalized to the case of first-order logic by Bidoit et al., is further generalized to this case. The fact that higher-order logic is powerful enough to express the indistinguishability relation is used to characterize behavioural satisfaction in terms of ordinary satisfaction, and to develop new methods for reasoning about specifications under behavioural semantics.

Observational specifications and the indistinguishability assumption

To establish the correctness of some software w.r.t. its formal specification is widely recognized as a difficult task. A first simplification is obtained when the semantics of an algebraic specification is defined as the class of all algebras which correspond to the correct realizations of the specification. A software is then declared correct if some algebra of this class corresponds to it. We approach this goal by defining an observational satisfaction relation which is less restrictive than the usual satisfaction relation. Based on this notion we provide an institution for observational specifications. The idea is that the validity of an equational axiom should depend on an observational equality, instead of the usual equality. We show that it is not reasonable to expect an observational equality to be a congruence. We define an observational algebra as an algebra equipped with an observational equality which is an equivalence relation but not necessarily a congruence. We assume that two values can be declared indistinguishable when it is impossible to establish they are different using some available observations. This is what we call the Indistinguishability Assumption. Since term observation seems sufficient for data type specifications, we define an indistinguishability relation on the carriers of an algebra w.r.t. the observation of an arbitrary set of terms. From a careful case study it follows that this requires to take into account the continuations of suspended evaluations of observation terms. Since our indistinguishability relation is not transitive, it is only an intermediate step to define an observational equality. Our approach is motivated by several examples.

On a quasi-set theory.

On a quasi-set theory.

New results in fuzzy clustering based on the concept of indistinguishability relation

The issue of validity in clustering is considered and a definition of fuzzy r-cluster that extends E. Ruspini's definition (1982) is proposed. This definition is based on an indistinguishability relation based on the concept of t-norm. The fuzzy r-cluster's metrical properties are studied through the dual concept of t-conorm that leads to G-pseudometrics. From the concept of G-pseudometric, fuzzy r-clusters and fuzzy cluster coverages are defined. The authors propose a measure of cluster validity based on the concept of fuzzy coverage. The basic idea of the approach presented is that the smaller the difference between the degrees of membership and the degrees of indistinguishability, the better the clustering.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>