Abstract
A Bernoulli factory is an algorithmic procedure for exact sampling of certain random variables having only Bernoulli access to their parameters. Bernoulli access to a parameter means the algorithm does not know p, but has sample access to independent draws of a Bernoulli random variable with mean equal to p. In this paper, we study the problem of Bernoulli factories for polytopes: given Bernoulli access to a vector for a given polytope , output a randomized vertex such that the expected value of the i-th coordinate is exactly equal to . For example, for the special case of the perfect matching polytope, one is given Bernoulli access to the entries of a doubly stochastic matrix and asked to sample a matching such that the probability of each edge be present in the matching is exactly equal to .
We show that a polytope admits a Bernoulli factory if and and only if is the intersection of with an affine subspace. Our construction is based on an algebraic formulation of the problem, involving identifying a family of Bernstein polynomials (one per vertex) that satisfy a certain algebraic identity on . The main technical tool behind our construction is a connection between these polynomials and the geometry of zonotope tilings.
We apply these results to construct an explicit factory for the perfect matching polytope. The resulting factory is deeply connected to the combinatorial enumeration of arborescences and may be of independent interest. For the k-uniform matroid polytope, we recover a sampling procedure known in statistics as Sampford sampling.11
Acknowledgements
The authors would like to thank Shaddin Dughmi, Jan Vondrak, Jason Hartline, and Bobby Kleinberg for several helpful comments.
Citation
Rad Niazadeh. Renato Paes Leme. Jon Schneider. "Combinatorial Bernoulli factories." Bernoulli 29 (2) 1246 - 1274, May 2023. https://doi.org/10.3150/22-BEJ1497