# Math Help - Dirichlet

1. ## Dirichlet

I am sorry for being rude in my previous thread, I would like to apologize and resubmit my question with all the correct notation and hopefully present it more clearer. My text is Elementary Number Theory by Jones and Jones. I do not understand Dirichlet product and series well at all. I am sure the definitions and lemmas in my text would be enough to tackle these problems, but I cannot seem to grasp them. Any help would be greatly appreciated, and I thank you for your time.

$k$ is a positive integer and $f,g$ and $h$ are arithmetic functions. $\hat{f}$ denotes the Dirichlet series for $f$, so

$\hat{f}(s)$ = $\sum_{n=1}^{\infty}{f(n)/n^s}$

Suppose $h$ is completely multiplicative. Define $f^{*k}$ inductively by $f^{*1}$= $f$and $f^{*(k+1)}$= $f*f^{*k}$.

Prove that:
(a) $(fh)*(gh) = (f*g)h$
(b) $(fh)*(gh)^{*k} = (f*g^{*k})h$
(c) $(fh)*h = (f*u)h$
(d) $(fh)*h^{*k} = (f*u^{*k})h$
(e) $h^{*k} = u^{*k}h$
(f) If $h$ is non-zero, then $(\mu^{*k}h) * h^{*k} =I$

(a) $(fh)*(gh) = (f*g)h$
How are you defining $fh$? Do you mean $f\circ h$? The reason why I am afraid to assume you mean $f\circ h$ is because $f,h$ are arithmetic functions, which can mean, $f,h:\mathbb{N}\mapsto \mathbb{C}$ and therefore composition would not make sense. Unless, here your arithmetic functions have their range as a subset of the naturals.

3. Are you saying that everything would make sense if it was in the form of $f\circ h$? Or not?

Are you saying that everything would make sense if it was in the form of $f\circ h$? Or not?
I am asking how do you define $fh$? I said it it reasonable to define $fh = f\circ h$ as longs as the ranges of $f,h$ are subsets of $\mathbb{N}$ otherwise the composition will not make sense. . So is it okay to assume that $f,g:\mathbb{N}\mapsto \mathbb{N}$?

5. Yes, they are subsets of $\mathbb{N}$. It is ok to assume what you stated. $f,g:\mathbb{N}\mapsto \mathbb{N}$ is correct. Sorry!

(a) $(fh)*(gh) = (f*g)h$
To show these functions are the same you need to show for any $n$ we have $[(f\circ h)*(g\circ h)](n) = [(f*g)\circ h](n)$.
$[(f\circ h)*(g\circ h)](n) = f(h(n))*g(h(n)) = \sum_{d|n} f(h(d))g(h(n/d))$.
$[(f*g)\circ h](n) = \sum_{d|n}f(h(n))g(h(n/d))$.
7. Thank you! I will try, but (b) and (d) are also confusing me with the $^{*k}$