Pebble game

In mathematics and computer science, a pebble game is a type of mathematical game played by placing "pebbles" or "markers" on a directed acyclic graph according to certain rules:A given step of the game consists of either placing a pebble on an empty vertex or removing a pebble from a previously pebbled vertex. A vertex may be pebbled only if all its predecessors have pebbles. The objective of the game is to successively pebble each vertex of G while minimizing the number of pebbles that are ever on the graph simultaneously.

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