A two-player game can be "solved" on several levels.
  1. Ultra-weak: In the weakest sense, solving a game means proving whether the first player will win, lose, or draw from the initial position, given perfect play on both sides.
  2. Weak: More typically, solving a game means providing an algorithm which secures a win for one player, or a draw for either, against any possible moves by the opponent, from the initial position only.
  3. Strong: The strongest sense of solution requires an algorithm which can produce perfect play from any position, i.e. even if mistakes have already been made on one or both sides. For a game with a finite number of positions, this is always possible with a powerful enough computer, by checking all the positions. However, there is the question of finding an efficient algorithm, or an algorithm that works on computers currently available.

Table of contents
1 Solved games
2 Partially solved games

Solved games

  • Awari (a game of the Mancala family)
    • Solved by Henri Bal and John Romein at the Free University in Amsterdam, Netherlands (2002). Either player can force the game into a draw.
  • Connect Four
    • Solved by both Victor Allis (1988) and James Allen (1989) independently. First player can force a win.
  • Gomoku
    • Solved by Victor Allis (1993) First player can force a win.
  • Hex
    • Completely solved (definition #3) by several computers for board sizes up to 6x6.
    • Jing Yang has demonstrated a winning strategy (definition #2) for board sizes 7x7, 8x8 and 9x9. [1]
    • John Nash showed that all board sizes are won for the first player (definition #1).
    • The discovery of an efficient general algorithm for an NxN board is unlikely as the problem has been shown to be NP-hard.
  • Nim
    • Completely solved for all starting configurations.
  • Nine men's morris
    • Solved by Ralph Gasser (1993). Either player can force the game into a draw. [1]
  • Pentominoes
    • Weakly solved (definition #2) by H. K. Orman. It is a win for the first player.
  • Qubic
    • Solved by Patashnik (1980).
  • Three Men's Morris
    • Trivially solvable. Either player can force the game into a draw
  • Tic-tac-toe
    • Trivially solvable. Either player can force the game into a draw

Partially solved games

  • Checkers
    • Endgames up to 8 pieces have been solved. Not all early-game positions have been solved, but almost all midgame positions are solved. Contrary to popular belief, Checkers is not completely solved.
  • Chess
    • Completely solved (definition #3) by retrospective computer analysis for all 2- to 5-piece endgames, counting the two kings as pieces. Also solved for pawnless 3-3 and most 4-2 endgames.
  • Go
    • Solved (definition #3) for board sizes up to 4x4. The 5x5 board is partially solved. [1] Humans usually play on a 19x19 board (which has an enormous complexity). Therefore solving the game appears very unlikely. On the other hand, a heuristic algorithmic solution might be achievable in the future. While not mathematically complete, it could give good results in practice, perhaps beating even the best human players.
  • Reversi
    • solved on a 4x4 and 6x6 board as a second player win.
  • mnk-games
    • It is trivial to show that the second player can never win. Almost all cases have been solved weakly for k <= 4. Some results are known for k = 5. The games are drawn for k >= 8.

See Also: Board game complexity

External link