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Define $\tau(n):=\sum_{d|n} 1$ by the number of divisors of a natural number $n$ and $\mu(n)$ by the Mobius function, which is given by

$$ \mu(n):=\begin{cases} (-1)^r &\mbox{ if }n=p_1p_2 \cdots p_r \mbox{ for distinct primes }p_1, \ldots, p_r \\\ 0 &\mbox{ otherwise.}\end{cases} $$

Dirichlet convolution of two arithmetic functions $f$ and $g$ is defined by:

$$ f \ast g := \sum_{d|n} f(d) g\left(\frac nd\right) $$

Also, define $\bf 1 \rm (n) := 1$ for all natural numbers $n$, and

$$\delta_1(n) := \begin{cases} 1 &\mbox{ if } n=1 \\\ 0&\mbox{ otherwise} \end{cases}$$

Which of the following formulas are correct?

Select ALL that apply.


$\bf 1 \rm \ast \bf 1 \rm = \tau$.


$\mu \ast \bf 1 \rm = \delta_1$.


$\mu \ast \tau = \bf 1 \rm$.


$\delta_1 \ast \bf 1 \rm = \bf 1 \rm$.

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