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Who keeps the ball in a five-person circle

Question

Five people, including you, sit in a circle, and you hold a ball at the start. Each second, whoever holds the ball picks one of three actions uniformly at random: pass it to the neighbour on the left, pass it to the neighbour on the right, or keep it. Play ends as soon as a holder decides to keep it. Find the chance that the game finishes with the ball in your hands.

Short answer

5/115/11. By symmetry only three states matter: you hold it (xx), a neighbour holds it (yy), or one of the two far players holds it (zz). Then x=13+23yx=\tfrac13+\tfrac23y, y=13x+13zy=\tfrac13x+\tfrac13z, z=13y+13zz=\tfrac13y+\tfrac13z. These give z=y/2z=y/2, y=2x/5y=2x/5 and finally x=5/11x=5/11.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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