Let $\displaystyle \mu $ be the Möbius function and $\displaystyle k,n\in\mathbb{N} $, evaluate
$\displaystyle \displaystyle \sum_{d^k\mid n} \mu(d) $
We can factor this as $\displaystyle \prod_{p^{km}||n}(\mu(1)+\mu(p)+\dots+\mu(p^m))$ where $\displaystyle p^{km} || n$ means that $\displaystyle p^{km}|n$ but $\displaystyle p^{k(m+1)}\nmid n$. Clearly the product is $\displaystyle 0$ unless $\displaystyle p^{0}||n$ for each $\displaystyle p$. Hence the sum is $\displaystyle 1$ if $\displaystyle n$ is not divisible by a $\displaystyle k$-power (other than $\displaystyle 1$), and $\displaystyle 0$ otherwise.
Well it's more that I'm assuming the reader will have no trouble convincing him/herself of it!
Each term in the sum is $\displaystyle \mu(d)$, where $\displaystyle d=p_1^{a_1}\cdots p_j^{a_j}$ and $\displaystyle d^k = p_1^{ka_1}\cdots p_j^{ka_j}$ is such that $\displaystyle d^k | n$. In the product I wrote, the term $\displaystyle \mu(d)$ will be obtained as $\displaystyle \mu(p_1^{a_1})\cdots \mu(p_j^{a_j}) = \mu(d)$ once the product is expanded. Conversely, each term in the expansion of the product can be found in the sum.