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Semantic of (Ongoing work with S. Hayashi, T. Coquand) Jouy-en-Josas, France, December 2004 Stefano Berardi, Semantic of Computation group C.S. Dept., Turin University, http://www.di.unito.it/~stefanohttp://www.di.unito.it/~stefano

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Abstract of the talk Backtracking is the possibility, for a given computation, to come back to a previous state and restart from it, forgetting everything took place after it. We introduce a new notion, 1-backtracking. It is a particular case of backtracking in which forgetting is irreversible. If we forget a state we can never restore it back. We introduce a game theoretical model for 1- backtracking. 2

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Games in Set Theory A game G = (T, R, turn, W E, W A ) between two players, E (Eloise) and A (Abelard), consists of: 1.a tree T with a father/child relation R. 2.A map turn :T { E, A }. 3.A partition (W E, W A ) of infinite branches of T. A play is any finite or infinite branch of T, starting from the root of T. In each node x of the branch, the player turn (x) must select a child of x in T, otherwise his opponent wins. If a play continues forever, the winner is E if the play is in W E, and it is A if the play is in W A.

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Games with 1-backtracking Given a game G, we define a game bck(G). In bck(G), the player p moving from the last position z of the play can come back to some previous position x, provided: x is before z in the tree of positions of G, and p moved from x. Then p can change the move y he did from x. xzyy … 1-backtracking x before z in G y previous movey new move

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Infinite 1-backtracking is loosing A player is allowed to backtrack to a given position x i of the play only finitely many times. A player backtracking infinitely many times to the same position x i looses. If E backtracks infinitely many times to some x i and A infinitely many times to some x j, the looser is the player backtracking infinitely many times to a position with smaller index.

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Removing 1-backtracking Fix any (finite or infinite) play = of bck(G). We can remove 1-backtracking from, by waiting that both players stop backtracking. The result is some canonical (finite or infinite) backtracking-free play (1) = of G. Definition of (1) runs as follows: 1.t 0 = initial position of G. 2.t n+1 = last child of t n in, provided t n has a last child in. Otherwise (1) ends.

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A formal definition of bck(G) Positions of bck(G). All finite successions over G, such that: (i) s 0 initial position of G (ii) any s i+1 is a child in G of some s j, with j i, s j ancestor in G of s i, and turn (s j ) = turn (s i ). Turn. The player on turn on a position of bck(G) is the player on turn on s n. Winner of a infinite play. The winner of an infinite play of bck(G) is the winner, in G, of the backtracking-free play (1).

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1-Backtracking and Limit Computable Mathematic Let A be any arithmetical formula in the connectives,,,. Let Tarski (A) be the Tarski game associated to A. Let LCM be Hayashis Limit Computable Mathematic (or Arithmetic with incremental learning). Theorem. A is true in LCM if and only E if has a recursive winning strategy on bck( Tarski (A)). 1-backtracking characterizes the set of formulas we can learn

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1-Backtracking and Recursive Degrees Let G any game either with alternating players, or with no infinite plays. Let p any player. Theorem. p has a winning strategy of recursive degree 1 for G if and only if p has a winning strategy of recursive degree 0 for bck(G). Assume we can define a winning strategy for G using an oracle for the Halting problem. Then: if we allow 1-backtracking, we can define a recursive winning strategy for the same G

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1- Backtracking and Excluded Middle Let A be any arithmetical formula in the connectives,,,. Let Tarski (A) be the Tarski game associated to A. Let HA be Intuitionistic Arithmetic. Let 1-EM be Excluded Middle for degree 1 formulas. Theorem. E has a recursive winning strategy for bck( Tarski (A)) if, and only if: HA + -rule + 1-EM |- A 1-backtracking characterizes the set of intuitionistic consequences of 1-EM.

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Cut-free 1-Backtracking As done by Coquand for general backtracking, we can define a cut-free version bck CF (G) of bck(G). bck CF (G) is the subgame of bck(G) in which Abelard cannot backtrack (cannot answer to a move which is not the previous one). It is much easier to define winning strategies for Eloise on the cut-free version of bck(G), because Abelard has an handicap. Every winning strategy for Eloise on bck CF (G) can be raised, in a canonical way, to a winning strategy for Eloise on bck(G).

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Iterating 1-Backtracking We defined the maps G| bck(G),bck CF (G) from games of Set Theory to games of Set Theory. By iteration, for any n N we can define bck n (G), bck CF n (G), the games with n-backtracking over G, with and without cuts. The difference between 1-backtracking and 2- backtracking is that, in 2-backtracking, forgetting is sometimes reversible (we may recover a previous state of the computation forgotten by 1- backtracking). By direct limit we can define bck (G), bck CF (G), for all ordinal.

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1-Backtracking and unlimited Backtracking For any game G with alternating players and all plays of length n, Coquand defined a game Coq(G). In Coq(G), Eloise has an unlimited backtracking over G. Abelard, instead, cannot backtrack: Coq(G) is cut-free. Theorem. bck CF (G) is a subgame of Coq(G), and conversely, any winning strategy in Coq(G) is a winning strategy in some bck CF (G). Unlimited backtracking can be obtained by iterating 1-backtracking.

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References T. Coquand, A semantics of evidence for classical arithmetic, Journal of Symbolic Logic 60, pp. 325-337, 1995. S. Hayashi and M. Nakata, Towards Limit Computable Mathematics, Types for Proofs and Programs, LNCS 2277, pp. 125-144, 2001.

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