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On Extensions of the Unanimous Vote Problem

Evan J. R. Brody, Haya Diwan, Lisa Hellerstein, Thomas Lidbetter

cs.DS Sep 29, 2026 · v1
Proofs of the inequalities behind the multiplicative and additive adaptivity-gap bounds are given as linked Lean files, generated with Claude and Aristotle.
The Unanimous Vote problem is to determine a fixed order in which to flip each of $n$ biased coins, where each coin can be flipped only once, such that the expected number of flips until seeing both a head and a tail (or flipping all coins) is minimized. Duman Keles et al. (arXiv:2510.16678 [cs.DS]) gave an $\mathcal{O}(n \log n)$-time algorithm for this problem. Extensions of the Unanimous Vote problem are a rich source of stochastic optimization problems. We focus on three: (1) a variant in which each coin can be flipped arbitrarily many times (a solution is thus an infinite sequence of coin choices), (2) a generalization with $d$-sided dice, that can each be rolled once, where dice must be rolled until two different outcomes are observed (or all dice have been rolled), and (3) a different generalization with $d$-sided dice, where dice must be rolled until all $d$ outcomes have been observed. For (1), we show that there is an optimal sequence which follows a simple greedy rule; the same rule only gives a 1-additive approximation for the original problem (arXiv:2510.16678 [cs.DS]). The rule also yields a correspondence between a particular optimal sequence and a related mechanical word, which we exploit to characterize the conditions under which this optimal sequence is periodic. We establish tight multiplicative and additive adaptivity gaps for this variant. For (2), we show that two different generalizations of the greedy rule from (arXiv:2510.16678 [cs.DS]) can be combined to obtain a PTAS. For (3), we give an $\mathcal{O}(\log d)$-approximation algorithm by reducing the problem to Submodular Ranking (arXiv:1007.2503 [cs.DS]); the same reduction technique can be used to yield approximation algorithms for other stochastic probing problems. Finally, we pose a number of related open questions.

The Unanimous Vote problem asks for a fixed order in which to flip n biased coins so that the expected number of flips until both a head and a tail appear is minimized. The paper studies three extensions: unlimited flips per coin, d-sided dice stopping at two distinct outcomes, and dice stopping once all d outcomes are seen (Finite Coupon Collection).

For the unlimited-flips variant, a greedy rule is shown to give an optimal infinite sequence. That sequence is linked to mechanical words to characterize when it is periodic. Adaptivity gaps are bounded, and the key inequalities behind these bounds are proved in Lean files (MulAdaptivityGap.lean, AddAdaptivityGap.lean). For the dice variants, generalized greedy rules and a reduction to Submodular Ranking are used.

The unlimited-flips variant has a tight multiplicative adaptivity gap of 1.2 and a tight additive adaptivity gap of 1/2. The d-ary Unanimous Vote problem admits a 1-additive approximation and a PTAS. Finite Coupon Collection admits an O(log d)-approximation.