How many digits are needed?

Abstract

Let $X_1,X_2,...$ be the digits in the base-$q$ expansion of a random variable $X$ defined on $[0,1)$ where $q\ge2$ is an integer. For $n=1,2,...$, we study the probability distribution $P_n$ of the (scaled) remainder $\sum_{k=n+1}^\infty X_qq^{n-k}$: If $X$ has an absolutely continuous CDF then $P_n$ converges in the total variation metric to Lebesgue measure on the unit interval; under certain smoothness conditions we establish exponentially fast convergence of $P_n$ and its PDF $f_n$; and we give examples of these results. The results are extended to the case of a multivariate random variable defined on a unit cube.Comment: 15 pages, 2 figure