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Back-to-back heads with coins of bias 1 over k

Question

Kevin tosses n≥2n\ge2 coins in order, where coin kk lands heads with probability 1k\tfrac1k. He earns $1\text{\char36}1 for every pair of adjacent coins that both show heads, with overlaps counting separately (so HHH earns $2\text{\char36}2). If p(n)p(n) is his expected earnings with nn coins, find lim⁡n→∞p(n)\lim_{n\to\infty}p(n) (enter −1-1 if it does not exist).

Short answer

11. By linearity, the expected payout is the sum over adjacent coin pairs (k,k+1)(k,k+1) of the chance both show heads, 1k⋅1k+1=1k−1k+1\tfrac{1}{k}\cdot\tfrac{1}{k+1}=\tfrac1k-\tfrac1{k+1}. The sum telescopes to p(n)=1−1np(n)=1-\tfrac1n, which tends to 11.

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

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