Paper 1, Section II, H

A coin-tossing game is played by two players, $A_{1}$ and $A_{2}$. Each player has a coin and the probability that the coin tossed by player $A_{i}$ comes up heads is $p_{i}$, where $0<p_{i}<1, i=1,2$. The players toss their coins according to the following scheme: $A_{1}$ tosses first and then after each head, $A_{2}$ pays $A_{1}$ one pound and $A_{1}$ has the next toss, while after each tail, $A_{1}$ pays $A_{2}$ one pound and $A_{2}$ has the next toss.

Define a Markov chain to describe the state of the game. Find the probability that the game ever returns to a state where neither player has lost money.

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