The Möbius function is the D4948: Integer function \begin{equation} \mathbb{N} \to \{ -1, 0, 1 \}, \quad n \mapsto \begin{cases} (-1)^k, \quad & n \text{ equals a product of } k \text{ distinct prime integers} \\ 0, \quad & \text{otherwise} \end{cases} \end{equation}