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We advance the program of connections between final coalgebras as sources of circularity in mathematics and fractal sets of real numbers. In particular, we are interested in the Sierpinski carpet, taking it as a fractal subset of the unit square. We construct a category of square sets and an endofunctor on it which corresponds to the operation of gluing copies of a square set along segments. We show that the initial algebra and final coalgebra exist for our functor, and that the final coalgebra is bi-Lipschitz equivalent to the Sierpinski carpet. Along the way, we make connections to topics such as the iterative construction of initial algebras as colimits, corecursive algebras, and the classic treatment of fractal sets due to Hutchinson., In Proceedings ACT 2021, arXiv:2211.01102

We are interested in connections between the theory of fractal sets obtained as attractors of iterated function systems and process calculi. To this end, we reinterpret Milner's expressions for processes as contraction operators on a complete metric space. When the space is, for example, the plane, the denotations of fixed point terms correspond to familiar fractal sets. We give a sound and complete axiomatization of fractal equivalence, the congruence on terms consisting of pairs that construct identical self-similar sets in all interpretations. We further make connections to labelled Markov chains and to invariant measures. In all of this work, we use important results from process calculi. For example, we use Rabinovich's completeness theorem for trace equivalence in our own completeness theorem. In addition to our results, we also raise many questions related to both fractals and process calculi.

The Vietoris space of compact subsets of a given Hausdorff space yields an endofunctor $\mathscr V$ on the category of Hausdorff spaces. Vietoris polynomial endofunctors on that category are built from $\mathscr V$, the identity and constant functors by forming products, coproducts and compositions. These functors are known to have terminal coalgebras and we deduce that they also have initial algebras. We present an analogous class of endofunctors on the category of extended metric spaces, using in lieu of $\mathscr V$ the Hausdorff functor $\mathcal H$. We prove that the ensuing Hausdorff polynomial functors have terminal coalgebras and initial algebras. Whereas the canonical constructions of terminal coalgebras for Vietoris polynomial functors takes $\omega$ steps, one needs $\omega + \omega$ steps in general for Hausdorff ones. We also give a new proof that the closed set functor on metric spaces has no fixed points.

We consider dynamic versions of epistemic logic as formulated in Baltag and Moss "Logics for epistemic programs" (2004). That paper proposed a logical language (actually families of languages parameterized by action signatures) for dynamic epistemic logic. It had been shown that validity in the language is Pi-1-1-complete, so there are no recursively axiomatized complete logical systems for it. In contrast, this paper proves a weak completeness result for the fragment without action iteration, and a strong completeness result for the fragment without action iteration and common knowledge. Our work involves a detour into term rewriting theory. The argument uses modal filtration, and thus we obtain the finite model property and hence decidability. We also give a translation of our largest language into PDL, thereby obtaining a second proof of decidability. The paper closes with some results on expressive power. These are mostly concerned with comparing the action-iteration-free language with modal logic augmented by transitive closure operators. We answer a natural question about the languages we obtain by varying the action signature: we prove that a logical language with operators for private announcements is more expressive than one for public announcements., This paper was intended to be the "journal" version of our 1998 paper "The logic of common knowledge, public announcements,and private suspicions." It was mainly written in 2004, with a few bibliographic additions coming a few years later

This paper obtains a completeness result for inequational reasoning with applicative terms without variables in a setting where the intended semantic models are the full structures, the full type hierarchies over preorders for the base types. The syntax allows for the specification that a given symbol be interpreted as a monotone function, or an antitone function, or both. There is a natural set of five rules for inequational reasoning. One can add variables and also add a substitution rule, but we observe that this logic would be incomplete for full structures. This is why the completeness result in this paper pertains to terms without variables. Since the completeness is already known for the class of general (Henkin) structures, we are interested in full structures. We present a completeness theorem. Our result is not optimal because we restrict to base preorders which have a weak completeness property: every pair of elements has an upper bound and a lower bound. To compensate we add several rules to the logic. We also present extensions and variations of our completeness result., 23 pages

This paper studies presentations of the Sierpinski gasket as a final coalgebra for functors on several categories of metric spaces with additional designated points. The three categories which we study differ on their morphisms: one uses short (non-expanding) maps, a second uses Lipschitz maps, and a third uses continuous maps. The functor in all cases is very similar to what we find in the standard presentation of the gasket as an attractor. We prove that the Sierpinski gasket itself is the final coalgebra of a naturally-occurring functor in the continuous setting. In the short setting, the final coalgebra exists but it is better described as the completion of the initial algebra, and this is not isomorphic to the Sierpinski gasket. In the Lipschitz setting, the final coalgebra does not exist. We determine the initial algebras in all three settings as well., 21 pages

We present a proof of Arrow's theorem from social choice theory that uses a fixpoint argument. Specifically, we use Banach's result on the existence of a fixpoint of a contractive map defined on a complete metric space. Conceptually, our approach shows that dictatorships can be seen as fixpoints of a certain process., In Proceedings TARK 2019, arXiv:1907.08335

The Initial Algebra Theorem by Trnkov�� et al. states, under mild assumptions, that an endofunctor has an initial algebra provided it has a pre-fixed point. The proof crucially depends on transfinitely iterating the functor and in fact shows that, equivalently, the (transfinite) initial-algebra chain stops. We give a constructive proof of the Initial Algebra Theorem that avoids transfinite iteration of the functor. For a given pre-fixed point A of the functor, it uses Pataraia���s theorem to obtain the least fixed point of a monotone function on the partial order formed by all subobjects of A. Thanks to properties of recursive coalgebras, this least fixed point yields an initial algebra. We obtain new results on fixed points and initial algebras in categories enriched over directed-complete partial orders, again without iteration. Using transfinite iteration we equivalently obtain convergence of the initial-algebra chain as an equivalent condition, overall yielding a streamlined version of the original proof., LIPIcs, Vol. 211, 9th Conference on Algebra and Coalgebra in Computer Science (CALCO 2021), pages 5:1-5:20

The Initial Algebra Theorem by Trnkov\'a et al.~states, under mild assumptions, that an endofunctor has an initial algebra provided it has a pre-fixed point. The proof crucially depends on transfinitely iterating the functor and in fact shows that, equivalently, the (transfinite) initial-algebra chain stops. We give a constructive proof of the Initial Algebra Theorem that avoids transfinite iteration of the functor. For a given pre-fixed point $A$ of the functor, it uses Pataraia's theorem to obtain the least fixed point of a monotone function on the partial order formed by all subobjects of $A$. Thanks to properties of recursive coalgebras, this least fixed point yields an initial algebra. We obtain new results on fixed points and initial algebras in categories enriched over directed-complete partial orders, again without iteration. Using transfinite iteration we equivalently obtain convergence of the initial-algebra chain as an equivalent condition, overall yielding a streamlined version of the original proof.

This paper studies Markov decision processes (MDPs) from the categorical perspective of coalgebra and algebra. Probabilistic systems, similar to MDPs but without rewards, have been extensively studied, also coalgebraically, from the perspective of program semantics. In this paper, we focus on the role of MDPs as models in optimal planning, where the reward structure is central. The main contributions of this paper are (i) to give a coinductive explanation of policy improvement using a new proof principle, based on Banach’s Fixpoint Theorem, that we call contraction coinduction, and (ii) to show that the long-term value function of a policy with respect to discounted sums can be obtained via a generalized notion of corecursive algebra, which is designed to take boundedness into account. We also explore boundedness features of the Kantorovich lifting of the distribution monad to metric spaces.

This paper studies fundamental questions concerning category-theoretic models of induction and recursion. We are concerned with the relationship between well-founded and recursive coalgebras for an endofunctor. For monomorphism preserving endofunctors on complete and well-powered categories every coalgebra has a well-founded part, and we provide a new, shorter proof that this is the coreflection in the category of all well-founded coalgebras. We present a new more general proof of Taylor's General Recursion Theorem that every well-founded coalgebra is recursive, and we study under which hypothesis the converse holds. In addition, we present a new equivalent characterization of well-foundedness: a coalgebra is well-founded iff it admits a coalgebra-to-algebra morphism to the initial algebra.

This is the proceedings of the Seventeenth conference on Theoretical Aspects of Rationality and Knowledge, 17-19 July 2019, Institut de Recherche en Informatique de Toulouse (IRIT), Toulouse University Toulouse, France. The mission of the TARK conferences is to bring together researchers from a wide variety of fields, including Artificial Intelligence, Cryptography, Distributed Computing, Economics and Game Theory, Linguistics, Philosophy, and Psychology, in order to further our understanding of interdisciplinary issues involving reasoning about rationality and knowledge.

Front Matter, Table of Contents, Preface, List of Authors

LIPIcs, Volume 35, CALCO'15, Complete Volume, LIPIcs, Vol. 35, 6th Conference on Algebra and Coalgebra in Computer Science (CALCO 2015), pages 0-0

We study finite state transduction of automatic and morphic sequences. Dekking proved that morphic sequences are closed under transduction and in particular morphic images. We present a simple proof of this fact, and use the construction in the proof to show that non-erasing transductions preserve a condition called alpha-substitutivity. Roughly, a sequence is alpha-substitutive if the sequence can be obtained as the limit of iterating a substitution with dominant eigenvalue alpha. Our results culminate in the following fact: for multiplicatively independent real numbers alpha and beta, if v is an alpha-substitutive sequence and w is a beta-substitutive sequence, then v and w have no common non-erasing transducts except for the ultimately periodic sequences. We rely on Cobham's theorem for substitutions, a recent result of Durand.

The final coalgebra for the finite power-set functor was described by Worrell who also proved that the final chain converges in ω+ω steps. We describe the step ω as the set of saturated trees, a concept equivalent to the modally saturated trees introduced by K. Fine in the 1970s in his study of modal logic. And for the bounded power-set functors P_λ, where λ is an infinite regular cardinal, we prove that the construction needs precisely λ+ω steps. We also generalize Worrell’s result to M-labeled trees for a commutative monoid M, yielding a final coalgebra for the corresponding functor ℳ_f studied by H.-P. Gumm and T. Schröder. We describe the final chain of the power-set functor by introducing the concept of i-saturated tree for all ordinals i, and then prove that for i of cofinality ω, the i-th step in the final chain consists of all i-saturated, strongly extensional trees.

We begin by discussing the history of quantum logic, dividing it into three eras or lives. The first life has to do with Birkhoff and von Neumann's algebraic approach in the 1930's. The second life has to do with attempt to understand quantum logic as logic that began in the late 1950's and blossomed in the 1970's. And the third life has to do with recent developments in quantum logic coming from its connections to quantum computation. We discuss our own work connecting quantum logic to quantum computation (viewing quantum logic as the logic of quantum registers storing qubits), and make some speculations about mathematics based on quantum principles., Comment: To appear in a special issue of Journal of Philosophical Logic

We combine ideas coming from several fields, including modal logic, coalgebra, and set theory. Modally saturated trees were introduced by K. Fine in 1975. We give a new purely combinatorial formulation of modally saturated trees, and we prove that they form the limit of the final omega-op-chain of the finite power-set functor Pf. From that, we derive an alternative proof of J. Worrell's description of the final coalgebra as the coalgebra of all strongly extensional, finitely branching trees. In the other direction, we represent the final coalgebra for Pf in terms of certain maximal consistent sets in the modal logic K. We also generalize Worrell's result to M-labeled trees for a commutative monoid M, yielding a final coalgebra for the corresponding functor Mf studied by H. P. Gumm and T. Schröder. We introduce the concept of an i-saturated tree for all ordinals i, and then prove that the i-th step in the final chain of the power set functor consists of all i-saturated trees. This leads to a new description of the final coalgebra for the restricted power-set functors Plambda (of subsets of cardinality smaller than lambda).

This paper provides a general account of the notion of recursive program schemes, studying both uninterpreted and interpreted solutions. It can be regarded as the category-theoretic version of the classical area of algebraic semantics. The overall assumptions needed are small indeed: working only in categories with "enough final coalgebras" we show how to formulate, solve, and study recursive program schemes. Our general theory is algebraic and so avoids using ordered, or metric structures. Our work generalizes the previous approaches which do use this extra structure by isolating the key concepts needed to study substitution in infinite trees, including second-order substitution. As special cases of our interpreted solutions we obtain the usual denotational semantics using complete partial orders, and the one using complete metric spaces. Our theory also encompasses implicitly defined objects which are not usually taken to be related to recursive program schemes. For example, the classical Cantor two-thirds set falls out as an interpreted solution (in our sense) of a recursive program scheme., this version includes the corrections from the corrigendum in Theoret. Comput. Sci. 403 (2008), 409-415

We present a generalization of modal logic to logics which are interpreted on coalgebras of functors on sets. The leading idea is that infinitary modal logic contains characterizing formulas. That is, every model-world pair is characterized up to bisimulation by an infinitary formula. The point of our generalization is to understand this on a deeper level. We do this by studying a fragment of infinitary modal logic which contains the characterizing formulas and is closed under infinitary conjunction and an operation called Δ. This fragment generalizes to a wide range of coalgebraic logics. Each coalgebraic logic is determined by a functor on sets satisfying a few properties, and the formulas of each logic are interpreted on coalgebras of that functor. Among the logics obtained are the fragment of infinitary modal logic mentioned above as well as versions of natural logics associated with various classes of transition systems, including probabilistic transition systems. For most of the interesting cases, there is a characterization result for the coalgebraic logic determined by a given functor. We then apply the characterization result to get representation theorems for final coalgebras in terms of maximal elements of ordered algebras. The end result is that the formulas of coalgebraic logics can be viewed as approximations to the elements of a final coalgebra.

This volume contains the Proceedings of the Fifth Workshop on Coalgebraic Methods in Computer Science (CMCS'2002). The Workshop was held in Grenoble, France on April 6--7 2002, as satellite event to ETAPS'2002.Over the last few years it has become clear that a great variety of state-based dynamical systems, like transition systems, automata, process calculi and class-based systems can be captured uniformly as coalgebras. The aim of the CMCS workshops is to bring together researchers with a common interest in the theory and application of coalgebras. The five CMCS volumes demonstrate that coalgebra is developing into a field of its own, presenting a deep mathematical foundation and a growing field of applications and interactions with various other fields, such as modal logic, category theory, dynamical systems, control systems, object-oriented and concurrent programming, formal systems specifications, algebra, analysis, combinatorics, and set theory.The papers in this volume were reviewed by the program committee: Jiri Adamek(Department of Computer Science, Technical University of Braunschweig)Alexandru Baltag(Department of Computer Science, Oxford University)H. Peter Gumm(Department of Mathematics and Computer Science, University of Marburg)Jesse Hughes(Department of Computer Science, University of Nijmegen)Bart Jacobs(Department of Computer Science, University of Nijmegen)Alexander Kurz(Department of Software Technology, CWI)Marina Lenisa(Department of Mathematics and Computer Science, University of Udine)Ugo Montanari(Department of Computer Science, University of Pisa)Larry Moss(Department of Mathematics, Indiana University)Ataru T. Nakagawa(SRA Key Technology Laboratory, Tokyo)John Power(Department of Computer Science, The University of Edinburgh)Horst Reichel(Institute of Theoretical Computer Science, Dresden University of Technology)Jan Rutten(Department of Software Technology, CWI)Several outside reviewers also assisted. CMCS received 20 submissions and accepted 15 of them. In addition, there were two invited speakers: Jose Meseguer and Luigi Santocanale. Their papers appear in this volume along with the 15 submitted contributions. We are grateful to everyone who sent us papers, and we regret that the length of the conference did not allow more papers to be presented.We thank the organizers of ETAPS'2002 for their help and encouragement. Special thanks to Rachid Echahed for his constant help with the organization of the workshop, and also to Mike Mislove for his work as a Managing Editor of the ENTCS series. Their efforts have been crucial for the success of CMCS'2002.September 1, 2002 Lawrence S. Moss

This volume contains the Proceedings of the joint meeting of two conferences: the sixth conference on Formal Grammar and the seventh on Mathematics of Language. The meeting was held just prior to the European Summer School in Logic, Language, and Information in August 2001.FGMOL'01 provided a forum for the presentation of new and original research on formal grammar and mathematical aspects of language, especially with regard to the application of formal methods to natural language analysis. Themes of interest included •formal and computational syntax, semantics, pragmatics, and phonology;•model-theoretic and proof-theoretic methods in linguistics;•constraint-based and resource-sensitive approaches to grammar;•foundational, methodological and architectural issues in grammar;•mathematical properties of linguistic frameworks;•theories and models of natural language processing and generation;•parsing theory; and•statistical and quantitative models of language.The meeting was a lively one. It featured the 23 papers in this ENTCS volume. It also had invited addresses by Jaako Hintikka (Boston University) and Aravind Joshi (University of Pennsylvania). There was a special symposium on Model Theoretic Syntax given by Patrick Blackburn (INRIA), Uwe Mönnich (Tübingen), Geoffrey Pullum (UC Santa Cruz), and James Rogers (Earlham). Finally, there was an ESSLLI workshop on Mathematics of Language held just after FGMOL'01.We are grateful to the organizers of ESSLLI for their encouragement of FGMOL, and also for supporting the workshop. We are also pleased to acknowledge Michael Mislove's help in preparing the files for the volume.The papers in this volume were reviewed by the following program committee: Anne Abeillé(Paris)Patrick Blackburn(INRIA)Gosse Bouma(Groningen)Mary Dalrymple(Xerox Parc)Elisabet Engdahl(Gotenborg)Nissim Francez(Haifa)Thilo Goetz(IBM)David Johnson(IBM)Mark Johnson(Brown)Gerhard Jäger(Utrecht)Aravind Joshi(Penn)Ruth Kempson(London)Geert-Jan Kruijff(Prague)Alain Lecomte(LORIA)Uwe Mönnich(Tübingen)Michael Moortgat(Utrecht)Richard Oehrle(Ashford)Mark-Jan Nederhof(Groningen)Owen Rambow(Cogentex)James Rogers(Earlham)Mark Steedman(Edinburgh)We are grateful to this committee for all of their help with the meeting. We also thank Geert-Jan Kruijff, the third member of our FGMOL Organizing Committee for all of his advice and hard work on the meeting. January 1, 2004Larry Moss and Dick Oehrle

We provide a game-theoretical solution to the Monty Hall problem, and its representation in IF-logic, a system of logic which is an extension of first-order logic with Independent Quantifiers. This solution has been originally proposed by Allen Mann, but here we focus more on the game-theoretical aspects and the philosophical significance.

In the 1980s, John Reynolds postulated that a parametrically polymorphic function is an ad-hoc polymorphic function satisfying a uniformity principle. This allowed him to prove that his set-theoretic semantics has a relational lifting which satisfies the Identity Extension Lemma and the Abstraction Theorem. However, his definition (and subsequent variants) have only been given for specific models. In contrast, we give a model-independent axiomatic treatment by characterising Reynolds’ definition via a universal property, and show that the above results follow from this universal property in the axiomatic setting.

The chapter reports a study of the effects of diagrammatic teaching of reasoning using an interactive multi-media learning environment and develops a theoretical framework for analysing such data.