Update docs/math/mobius.md

因此

$$

\operatorname g(i\cdot p_j)=\operatorname g(i)+(\operatorname g(i)-\operatorname g(\frac{i}{p_j}))\cdot p_j^2

\operatorname g(i\cdot p_j)=\operatorname g(i)+\left (\operatorname g(i)-\operatorname g(\frac{i}{p_j})\right )\cdot p_j^2

$$

 **时间复杂度** ： $\Theta(n+T)$