Compositions of Random Functions on a Finite Set

Abstract

If we compose sufficiently many random functions on a finite set, then the composite function will be constant. We determine the number of compositions that are needed, on average. Choose random functions $f_1, f_2,f_3,\dots $ independently and uniformly from among the $n^n$ functions from $[n]$ into $[n]$. For $t>1$, let $g_t=f_t\circ f_{t-1}\circ \cdots \circ f_1$ be the composition of the first $t$ functions. Let $T$ be the smallest $t$ for which $g_t$ is constant(i.e. $g_t(i)=g_t(j)$ for all $i,j$). We prove that $E(T)\sim 2n$ as $n\rightarrow\infty$, where $E(T)$ denotes the expected value of $T$.