2
$\begingroup$

Let $s(n) = n(n + 1)(2n + 1)/6$ the sum of squares.

How can we prove that $(\mu * s)(n) = \tfrac{1}{3} n \varphi(n) + \tfrac{1}{6} \prod_{p|n}(1-p)$?

  • $\mu(1) = 1$
  • $\mu(a^2b) = 0$
  • $\mu(p_1 p_2 \cdots p_k) = (-1)^k$
  • 0
    What I said previously would be the a strategy, but your identity is just not true. Plug in $n=1$ or $n=2$.2011-04-24

2 Answers 2

2

You cant prove this because it is false. Let $n=1.$ Then $\mu*s(1)=\mu (1)s(1)=1$ while $\frac{1}{3}1\cdot 1+\frac{1}{6}1=\frac{1}{2}.$ Similarly if $n=2$ the left hand side is $3$ while the right hand side is $1$. Lastly when $n=3$, the left hand side is $8$ while the right hand side is $\frac{10}{3}.$ The reason why they can't be equal is that $s(n)$ is not a multiplicative function.

  • 0
    Since both sides are not multiplicative functions that would not be a problem in itself.2011-04-24
3

Just write $s(n)$ out as a polynomial. $s(n)=n^3/3 + n^2/2 + n/6$.

Then determine the convolutions of $n^3$, $n^2,$ and $n$ with $\mu$. These are easier to compute because $n^k$ is a multiplicitive function, so we only need to determine the values for prime powers.

We can figure out $m^k*\mu$ by evaluating on $p^\alpha$:

$(m^k*\mu)(p^\alpha) = p^{k\alpha} - p^{k(\alpha-1)}=\phi(p^\alpha) p^{(k-1)(\alpha-1)}\frac{p^k-1}{p-1}$

So if $n=p_1^{\alpha_1}...p_t^{\alpha_t}$ then:

$(m^k * \mu)(n) = \phi(n) \, \prod_{i=1}^{t} {p_i^{(\alpha_i-1)(k-1)}\frac{p^k-1}{p-1}}$

Let $n_0 = p_1p_2...p_t$. Then

$(m^k * \mu) = \phi(n) (\frac{n}{n_0})^{k-1} \prod_i \frac{p^k-1}{p-1}$

So $(s*\mu)(n) = {\frac{\phi(n)}{6}}[1 + 3\frac{n}{n_0}\prod(1+p) + 2(\frac{n}{n_0})^2\prod(1+p+p^2)]$ $= \frac{\phi(n)}{6}[1 + 3n \prod(1+\frac{1}{p}) + 2n^2 \prod (1+\frac{1}{p}+\frac{1}{p^2})]$

This has as a lower bound $\frac{\phi(n)n^2}{3}$, and so your formula is wildly incorrect.

  • 0
    Thanks for this, I think I must have made a mistake in my earlier work.. trying to correct it.2011-04-24