\[\mu \left( n \right) = \left\{ \begin{array}{l} 1,{\text{ }}n = 1 \hfill \\ {\left( { - 1} \right)^k},{\text{ }}n = {p_1}{p_2}...{p_k} \hfill \\ 0,{\text{ }}{p^2}|n \hfill \\ \end{array} \right.\]