## Simulating a coin with irrational bias using rational arithmetic

Posted in Books, Statistics with tags , , , , , , on December 17, 2020 by xi'an

An arXived paper by Luis Mendo adresses the issue of simulating coins with irrational probabilities from a fair coin, somehow connected with one of the latest riddles. Where I realised only irrational coins could be simulated in a fixed and finite number of throws! The setting of the paper is however similar to the one of a Bernoulli factory in that an unlimited number of coins can be generated (but it relies on a  fair coin). And the starting point is a series representation of the irrational ζ as a sum of positive and rational terms. As well as an earlier paper by the Warwick team of Łatuszyński et al. (2011). The solution is somewhat anticlimactic in that the successive draws of the fair coin lead to a sequence of intervals with length divided by 2 at each step. And stopping when a certain condition is met, requiring some knowledge on the tail error of the series. The paper shows further that the number of inputs used by its algorithm has an exponential tail. The examples provided therein are Euler’s constant

$\gamma =\frac{1}{2} + \sum_{i=1}^\infty \frac{B(i)}{2i(2i+1)(2i+2)}$

where B(j) is the number of binary digits of j, and π/4 which can be written as an alternating series. An idle question that came to me while reading this paper is the influence of the series chosen to represent the irrational ζ as it seems that a faster decrease in the series should lead to fewer terms being used. However, the number of iterations is a geometric random variable with parameter 1/2, therefore the choice of the series curiously does not matter.