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The "Ercules and the Hydra" problem, as found in "L. Kirby and J. Paris. Accessible independence results for peano arithmetic. London Mathematical Society, 4:285 293, 1982.".

It is easily solved with

Using transfinite induction, but it is possible to show that Hercules will always kill the hydra (with a finite number of blows) regardless of the strategy chosen to chop off hydra's heads. Moreover, this fact is not provable within Peano Arithmetic.

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The "Ercules and the Hydra" problem, as found in "L. Kirby and J. Paris. Accessible independence results for peano arithmetic. London Mathematical Society, 4:285 293, 1982.".

It is easily solved with transfinite induction, but it is not provable within Peano Arithmetic.