A D993: Real number $x \in \mathbb{R}$ is a standard Cantor number if and only if \begin{equation} \exists \, a_1, a_2, a_3, \ldots \in \{ 0, 2 \} : x = \sum_{n \in \{ 1, 2, 3, \ldots \}} \frac{a_n}{3^n} \end{equation}