This paper compares two different styles of reasoning with inductively defined predicates, each style being encapsulated by a corresponding sequent calculus proof system. The first system supports traditional proof by induction, with induction rules formulated as sequent rules for introducing inductively defined predicates on the left of sequents. We show this system to be cut-free complete with respect to a natural class of Henkin models; the eliminability of cut follows as a corollary. The second system uses infinite (non-well-founded) proofs to represent arguments by infinite descent. In this system, the left rules for inductively defined predicates are simple case-split rules, and an infinitary, global condition on proof trees is required to ensure soundness. We show this system to be cut-free complete with respect to standard models, and again infer the eliminability of cut. The second infinitary system is unsuitable for formal reasoning. However, it has a natural restriction to proofs given by regular trees, i.e. to those proofs representable by finite graphs. This restricted “cyclic ” system subsumes the first system for proof by induction. We conjecture that the two systems are in fact equivalent, i.e., that proof by induction is equivalent to regular proof by infinite descent.

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We present a process semantics for the purely additive fragment of linear logic in which formulas denote protocols and (equivalence classes of) proofs denote multi-channel concurrent processes. The polycategorical model induced by this process semantics is shown to be equivalent to the free polycategory based on the syntax (i.e., it is full and faithfully complete). This establishes that the additive fragment of linear logic provides a semantics of concurrent processes. Another property of this semantics is that it gives a canonical representation of proofs in additive linear logic.

Abstract. Using coalgebraic methods, we extend Conway’s original theory of games to include infinite games (hypergames). We take the view that a play which goes on forever is a draw, and hence rather than focussing on winning strategies, we focus on non-losing strategies. Infinite games are a fruitful metaphor for non-terminating processes, Conway’s sum of games being similar to shuffling. Hypergames have a rather interesting theory, already in the case of generalized Nim. The theory of hypergames generalizes Conway’s theory rather smoothly, but significantly. We indicate a number of intriguing directions for future work. We briefly compare infinite games with other notions of games used in computer science.

Abstract. First, we present Conway’s theory of games under an algebraic perspective. Then, using coalgebraic methods, we extend Conway’s theory to infinite games (hypergames). We take the view that a play which goes on forever is a draw, and hence rather than focussing on winning strategies, we focus on non-losing strategies. Infinite games are a fruitful metaphor for non-terminating processes, Conway’s sum of games being similar to shuffling. Hypergames have a rather interesting theory, already in the case of impartial hypergames, for which we give a compositional semantics, in terms of a generalized Grundy-Sprague function and a system of generalized Nim games. Equivalences and congruences on games and hypergames are discussed. The theory of hypergames generalizes Conway’s theory rather smoothly, but significantly. We indicate a number of intriguing directions for future work. We briefly compare infinite games with other notions of games used in computer science. 1.

Abstract. Parity games are combinatorial representations of closed Boolean µ-terms. By adding to them draw positions, they have been organized by Arnold and one of the authors [1, 2] into a µ-calculus [3]. As done by Berwanger et al. [4, 5] for the propositional modal µ-calculus, it is possible to classify parity games into levels of hierarchy according to the number of fixed-point variables. We ask whether this hierarchy collapses w.r.t. the standard interpretation of the games µ-calculus into the class of all complete lattices. We answer this question negatively by providing, for each n ≥ 1, a parity game Gn with these properties: it unravels to a µ-term built up with n fixed-point variables, it is semantically equivalent to no game with strictly less than n − 2 fixed-point variables. 1

Taking the view that infinite plays are draws, we study Conway non-terminating games and non-losing strategies. These admit a sharp coalgebraic presentation, where non-terminating games are seen as a final coalgebra and game contructors, such as disjunctive sum, as final morphisms. We have shown, in a previous paper, that Conway’s theory of terminating games can be rephrased naturally in terms of game (pre)congruences. Namely, various conceptually independent notions of equivalence can be defined and shown to coincide on Conway’s terminating games. These are the equivalence induced by the ordering on surreal numbers, the contextual equivalence determined by observing what player has a winning strategy, Joyal’s categorical equivalence, and, for impartial games, the denotational equivalence induced by Grundy semantics. In this paper, we discuss generalizations of such equivalences to non-terminating games and non-losing strategies. The scenario is even more rich and intriguing in this case. In particular, we investigate efficient characterizations of the contextual equivalence,