Players A and B are playing a game where they take turns flipping a biased coin, with p probability of landing on heads (and winning). Player A starts the game, and then the players pass the coin back and forth until one person flips heads and wins. What is the probability that A wins?
My solution is to do the following:
- Treat the problem as an infinite sum
- We can get the odd numbers by doubling and adding one to any integer
- We can then reduce the problem to an infinite geometric series with $a = 1$ and $r = (1-p)^2$
$$ \sum_{i \text { odd}} \Bigl(1-p\Bigr)^ip= p\sum_{i =0}^{\infty} \Bigl({1 - p}\Bigr)^{2i+1}= p(1-p)\sum_{i =0}^{\infty} \Bigl({1\over (1-p)^{2}}\Bigr)^{ i }={1\over 1 - (1-p)^{2}}\cdot{p(1-p)}. $$
Does this seem correct?
This is based on the work in this thread, which asks for a solution for the specific instance of an unbiased coin. However, I am trying to solve for the general solution when the coin may be biased.