where each \(b\) is \(0\) or \(1\). Then each binary number may be viewed as the characteristic function of a finite subset of natural numbers. (On the left, there are infinitely many zeros, as in \(\ldots000111\), which are omitted.) For a natural number \(n\), \(e_n\) denotes the corresponding finite set of natural numbers. (E.g.,  \(e_7=\{0,1,2\}\).)