Coin flipping and waiting times paradoxes: Why fair coins are exceptional
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arXiv
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| Main Authors: | , |
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| Format: | Preprint |
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2026
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| _version_ | 1866914275392487424 |
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| author | Riis, Søren Paterson, Mike |
| author_facet | Riis, Søren Paterson, Mike |
| contents | Penney's Ante exhibits non-transitivity when two target strings race to appear in a shared stream of coin tosses. We study instead independent string races, where each player observes their own independent and identically distributed (i.i.d.) coin/die stream (possibly biased), and the winner is the player whose target appears first (under an explicit tie convention). We derive compact generating-function formulas for waiting times and a Hadamard-generating-function calculus for head-to-head odds. Our main theorem shows that for a fair -sided die, stochastic dominance induces a total pre-order on all strings, ordered by expected waiting time. For binary coins, we also prove a converse: total comparability under stochastic dominance characterises the fair coin (), and any bias yields patterns whose waiting times are incomparable under stochastic dominance. In contrast, bias allows both (i) reversals between mean waiting time and win probability and (ii) non-transitive cycles; we give explicit examples and certified computational classifications for short patterns. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2601_16580 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Coin flipping and waiting times paradoxes: Why fair coins are exceptional Riis, Søren Paterson, Mike Probability 60C05, 60G40, 68R15 G.2.1; G.3 Penney's Ante exhibits non-transitivity when two target strings race to appear in a shared stream of coin tosses. We study instead independent string races, where each player observes their own independent and identically distributed (i.i.d.) coin/die stream (possibly biased), and the winner is the player whose target appears first (under an explicit tie convention). We derive compact generating-function formulas for waiting times and a Hadamard-generating-function calculus for head-to-head odds. Our main theorem shows that for a fair -sided die, stochastic dominance induces a total pre-order on all strings, ordered by expected waiting time. For binary coins, we also prove a converse: total comparability under stochastic dominance characterises the fair coin (), and any bias yields patterns whose waiting times are incomparable under stochastic dominance. In contrast, bias allows both (i) reversals between mean waiting time and win probability and (ii) non-transitive cycles; we give explicit examples and certified computational classifications for short patterns. |
| title | Coin flipping and waiting times paradoxes: Why fair coins are exceptional |
| topic | Probability 60C05, 60G40, 68R15 G.2.1; G.3 |
| url | https://arxiv.org/abs/2601.16580 |