A Dynamic Epistemic Logic Analysis of Equality Negation and Other Epistemic Covering Tasks$,$$
A Dynamic Epistemic Logic Analysis of Equality Negation and other Epistemic Covering TasksI,II Hans van Ditmarscha, Eric´ Goubaultb, Marijana Lazi´cc, J´er´emy Ledentd, Sergio Rajsbaume aUniversit´ede Lorraine, CNRS, LORIA, F-54000 Nancy, France bLIX, CNRS, Ecole´ Polytechnique, Institute Polytechnique de Paris, France cTechnical University of Munich, Germany dDepartment of Computer and Information Sciences, University of Strathclyde, UK eInstituto de Matem´aticas, UNAM, Mexico Abstract In this paper we study the solvability of the equality negation task in a simple wait- free model where two processes communicate by reading and writing shared variables or exchanging messages. In this task, the two processes start with a private input value in the set f0; 1; 2g, and after communicating, each one must decide a binary output value, so that the outputs of the processes are the same if and only if the input values of the processes are different. This task is already known to be unsolvable; our goal here is to prove this result using the dynamic epistemic logic (DEL) approach introduced by Goubault, Ledent and Rajsbaum in GandALF 2018. We show that in fact, there is no epistemic logic formula that explains why the task is unsolvable. Furthermore, we observe that this task is a particular case of an epistemic covering task. We thus establish a connection between the existing DEL framework and the theory of covering spaces in topology, and prove that the same result holds for any epistemic covering task: no epistemic formula explains the unsolvability. Keywords: Dynamic Epistemic Logic, distributed computing, equality negation, combinatorial topology, covering spaces.
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