Geodesic
Hercules versus Hidden Hydra Helper
Commentationes Mathematicae Universitatis Carolinae, Tome 32 (1991) no. 4, pp. 731-741.

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L. Kirby and J. Paris introduced the Hercules and Hydra game on rooted trees as a natural example of an undecidable statement in Peano Arithmetic. One can show that Hercules has a ``short'' strategy (he wins in a primitively recursive number of moves) and also a ``long'' strategy (the finiteness of the game cannot be proved in Peano Arithmetic). We investigate the conflict of the ``short'' and ``long'' intentions (a problem suggested by J. Ne{\v s}et{\v r}il). After each move of Hercules (trying to kill Hydra fast) there follow $k$ moves of Hidden Hydra Helper (making the same type of moves as Hercules but trying to keep Hydra alive as long as possible). We prove that for $k=1$ Hercules can make the game short, while for $k\geq 2$ Hidden Hydra Helper has a strategy for making the game long.
Classification : 03B25, 03F30, 05C05, 90D46, 90D99
Keywords: rooted tree; unprovability; Kirby--Paris Theorem 
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     title = {Hercules versus {Hidden} {Hydra} {Helper}},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {731--741},
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Matoušek, Jiří; Loebl, Martin. Hercules versus Hidden Hydra Helper. Commentationes Mathematicae Universitatis Carolinae, Tome 32 (1991) no. 4, pp. 731-741. http://geodesic.mathdoc.fr/item/CMUC_1991__32_4_a15/