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Games on Posets - CiE 2008

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Games on Posets - CiE 2008

  1. 1. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Games on Posets Craig Wilson McMaster University June 22, 2008 Based on the paper “On the Complexity of Computing Winning Strategies for Finite Poset Games” by Michael Soltys and Craig Wilson Craig Wilson McMaster University Games on Posets
  2. 2. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Outline 1 Background Information 2 Translating Chomp to Geography 3 Chomp and Proof Complexity 4 Concluding Remarks 5 References Craig Wilson McMaster University Games on Posets
  3. 3. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Context of Poset Games Posets Poset Games The Game of Chomp Context of Poset Games Simple to describe Computing winning strategies appears intractable in all but simplest cases Proof of 1st player having a winning strategy does not immediately yield a feasible algorithm for computing the strategy Craig Wilson McMaster University Games on Posets
  4. 4. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Context of Poset Games Posets Poset Games The Game of Chomp Partially Ordered Sets (Posets) Set U together with ordering relation satisfies following properties: Anti-Symmetry: If a b, then b a Transitivity: If a b and b c, then a c If a b and b a, then a||b (incomparable) Craig Wilson McMaster University Games on Posets
  5. 5. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Context of Poset Games Posets Poset Games The Game of Chomp (Finite) Poset Games [1] Games between 2 players From finite Poset (U, ), Poset Game (A, ) is played as follows: 1 Initialize A = U 2 Select an x ∈ A, remove all y ∈ A such that x y 3 Game ends when A = ∅, player unable to select an element loses A round is a sequence of two consecutive moves (first player, then second) Craig Wilson McMaster University Games on Posets
  6. 6. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Context of Poset Games Posets Poset Games The Game of Chomp Chomp[2] x Special case of poset game. Ordering relation can be thought of as a chocolate bar. Don’t eat the poisoned square! Craig Wilson McMaster University Games on Posets
  7. 7. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Context of Poset Games Posets Poset Games The Game of Chomp More Formally. . . Set of pairs (“cells”) {(i, j) |1 ≤ i ≤ n, 1 ≤ j ≤ m} Select pair (i0, j0) Remove all (i, j) such that i ≥ i0 and j ≥ j0 Player left with only (1, 1) loses Craig Wilson McMaster University Games on Posets
  8. 8. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Context of Poset Games Posets Poset Games The Game of Chomp Chomp Configurations Represent as binary strings X: x Figure: Chomp configuration with X = 00101101 1s delimit rows, 0s indicate difference in rows lengths. Craig Wilson McMaster University Games on Posets
  9. 9. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Chomp ∈ PSPACE (Part 1) Geography Gadget Construction Challenges Translating Chomp to Geography Simple, direct polynomial-time translation from Poset Games to Geography, which shows Poset Games ∈ PSPACE Translation from restricted form of Geography to Chomp also attempted Limitations of poset games revealed - not PSPACE-complete? Craig Wilson McMaster University Games on Posets
  10. 10. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Chomp ∈ PSPACE (Part 1) Geography Gadget Construction Challenges Mathematically... GeneralizedGeography(GG): GG = { G, s | Player 1 has a winning strategy for the Generalized Geography game played on graph G starting at node s} Craig Wilson McMaster University Games on Posets
  11. 11. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Chomp ∈ PSPACE (Part 1) Geography Gadget Construction Challenges Graph Construction: Part 1 x yz Figure: Base Construction of a poset game in Geography Craig Wilson McMaster University Games on Posets
  12. 12. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Chomp ∈ PSPACE (Part 1) Geography Gadget Construction Challenges Complications... Once a node x ∈ G has been visited, must not be able to visit any y such that x y Need system of checks and balances x x1x2 y x4 x3z incoming moves incoming challenges outgoing moves Figure: 5-node gadget Craig Wilson McMaster University Games on Posets
  13. 13. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Chomp ∈ PSPACE (Part 1) Geography Gadget Construction Challenges Gadget Construction x x1x2 y x4 x3z incoming moves incoming challenges outgoing moves Figure: Construction of the 5-node gadget Craig Wilson McMaster University Games on Posets
  14. 14. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Chomp ∈ PSPACE (Part 1) Geography Gadget Construction Challenges Preventing Illegal Moves Additional nodes prevent players from making illegal moves. If player moves to node that “shouldn’t exist”, they will lose. Challenges to legal moves also result in a loss. Craig Wilson McMaster University Games on Posets
  15. 15. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Chomp ∈ PSPACE (Part 1) Geography Gadget Construction Challenges Example: Challenging an Illegal Move x x1x2 y x4 x3z incoming moves incoming challenges outgoing moves Figure: Challenging an illegal move Craig Wilson McMaster University Games on Posets
  16. 16. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Chomp and Proof Complexity Second proof of Chomp ∈ PSPACE Use theorems of W1 1 to show the existence of a winning strategy ∈ PSPACE Introduce segments X[i] of a configuration string X: X = X[1] 01 X[2] 10 X[3] 11 X[4] 00 Craig Wilson McMaster University Games on Posets
  17. 17. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Proof Complexity Main idea: ΓC ∀x∃yα(x, y) ∃f ∈ C such that α(x, f (x)) Different bounded arithmetic theories capture different classes Craig Wilson McMaster University Games on Posets
  18. 18. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Variables of a Different “Sort” Three types of variables called sorts: 1 Natural numbers (x, y, z, . . . ) 2 Strings (X, Y , Z, . . . ) 3 Sets of strings (X, Y, Z, . . . ) Concerned with formulas from class ΣB 1 : (∃X) (∀Y ) (∃Z) . . . (∀y) A (X, Y , Z, . . . , y) Craig Wilson McMaster University Games on Posets
  19. 19. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Description of W1 1 [3] Third-sorted theory for reasoning in PSPACE Symbols taken from set L3 A = {0, 1, +, ×, ||2, ∈2, ∈3, ≤, =} Axioms [3]: B1. x + 1 = 0 B7. (x ≤ y ∧ y ≤ x) → x = y B2. (x + 1 = y + 1) → x = y B8. x ≤ x + y B3. x + 0 = x B9. 0 ≤ x B4. x + (y + 1) = (x + y) + 1 B10. x ≤ y ∨ y ≤ x B5. x × 0 = 0 B11. x ≤ y ↔ x < y + 1 B6. x × (y + 1) = (x × y) + x B12. x = 0 → ∃y ≤ x(y + 1 = x) L1. X(y) → y < |X| L2. y + 1 = |X| → X(y) SE. [|X| = |Y | ∧ ∀i < |X|(X(i) ↔ Y (i))] → X = Y Craig Wilson McMaster University Games on Posets
  20. 20. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Achieving the Goal - Step 1 Want to give formula Φ(X, n, m) which decides whether X is valid Chomp game of size n × m X is string of length (n × m)(n + m), with (n × m) segments Φ is conjunction of three formulas: φinit, φfinal, and φmove Craig Wilson McMaster University Games on Posets
  21. 21. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Formulas φinit and φfinal φinit asserts X[1] to be the initial configuration of the game: φinit(X[1] , n, m) = ∀i ≤ (n + m)((i > m) → X[1] (i) = 1) φfinal asserts X[n×m] to be the final configuration of the game: φfinal(X[n×m] , n, m) = ∀i ≤ (n + m)((i > n) → X[n×m] (i) = 0) Craig Wilson McMaster University Games on Posets
  22. 22. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy The φyields formula φyields asserts that each segment of X can be obtained from one legal move on the previous one: φmove(X, n, m) = ∀i < (n × m)(X[i] “yields” X[i+1] ) Defining “yields” is where the fun begins! Attempt to discern what square(s) could have been played between X[i] and X[i+1] Ensure differences between configs correspond to played square Craig Wilson McMaster University Games on Posets
  23. 23. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Complete Yields Formula (∃j ≤ NumOnes)(∃k ≤ NumZeros)[F0(1, k, X) < F1(1, j, X) ∧(∃p < |X[i] |)(∃q ≤ |X[i] |)(p = F0(1, k − 1, X[i] ) + 1 ∧ q = F1(p, j, X[i] )) ∧(∀r ≤ |X|)[(r < p ∨ r > q → X[i+1] (r) = X[i] (r)) ∧(r < p + j → X[i+1] (r) = 1) ∧(r ≥ p + j → X[i+1] (r) = 0)]] Where NumOnes = |X[i] | − numones(1, X[i] ) NumZeroes = numones(1, X[i] ) Craig Wilson McMaster University Games on Posets
  24. 24. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Achieving the Goal, Step 2 Strategy function S from configurations to configurations Formula for either player having winning strategy: ∀C∃S [WinP1 (S, C) ∨ WinP2 (S, C)] where WinP1 (S, C) is a ΣB 1 formula Manipulate this to get formula for first player having a winning strategy: (C = 0m 1n ) → ∃S [WinP1 (S, C)] Craig Wilson McMaster University Games on Posets
  25. 25. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References The Goal Bounded Arithmetic Description of W1 1 Construction of Formula Φ(X, n, m) Existence of Winning Strategy Applying the Witnessing Theorem With W1 1 (C = 0m 1n ) → ∃S [WinP1 (S, C)] we have that the function for computing the winning strategy is in PSPACE. Craig Wilson McMaster University Games on Posets
  26. 26. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References Concluding Remarks Is the problem of computing winning strategies for Poset Games PSPACE-complete? Craig Wilson McMaster University Games on Posets
  27. 27. Background Information Translating Chomp to Geography Chomp and Proof Complexity Concluding Remarks References References I Steven Byrnes. Poset Game Periodicity. Integers: Electronic Journal of Combinatorial Number Theory, 3(G03), November 2003. David Gale. A Curious Nim-Type Game. The American Mathematical Monthly, 81(8):876–879, October 1974. Alan Skelley. A Third-Order Bounded Arithmetic Theory for PSPACE. In CSL, pages 340–354, 2004. Craig Wilson McMaster University Games on Posets

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