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This question is about applications of abelian characters to odd perfect numbers:

Context and Definitions:

Let $n$ be a natural number and $D_n$ be the set of divisors. We can make this set to a ring by observing that each divisor $d$ has

$$0 \le v_p(d) \le v_p(n)$$

Hence we can add two divisors $d,e$ by setting:

$$d \oplus e := \prod_{p | n} p^{v_p(d)+v_p(e) \mod (v_p(n)+1)}$$

and similarily we can multiply them by setting: $$d \otimes e := \prod_{p | n} p^{v_p(d) \cdot v_p(e) \mod (v_p(n)+1)}$$

Then, if $n = p_1^{a_1} \cdots p_r^{a_r}$, this ring will be isomorphic to the ring

$$\mathbb{Z}/(a_1+1) \times \cdots \times \mathbb{Z}/(a_r+1)$$

If $n$ is squarefree, than this reduces to :

$$d\oplus e = \frac{de}{\gcd(d,e)^2}$$

$$d\otimes e = \gcd(d,e)$$

Let $\phi_n(a):= \sum_{p|a} \frac{v_p(a)}{\sqrt{v_p(n)+1}}e_p$, then:

$$ \langle \phi_n(a),\phi_n(b) \rangle = \sum_{p|\gcd(a,b)} \frac{v_p(a)v_p(b)}{v_p(n)+1} =:K_n(a,b)$$

Set

$$\chi_n(d,e) := \chi^{(n)}_d(e):=\chi^{(n)}_e(d):=\exp\left(2 \pi \sqrt{-1} K_n(d,e) \right )$$

. Fix some natural number $n$ and let $\chi_d=:\chi^{(n)}_d$:

$$C(n) := \{ d:d|n, \chi_d = \bar{\chi}_d \}$$

These (two) questions came after researching the set $C(n)$ empirically with SageMath:

  1. $C(d) \cdot C(\frac{n}{d}) =^? C(n) \forall d \in \mathbf{U}(n):= \{d:d|n, \gcd(d,n/d)=1\}$

  2. $C(p^a) =^? \{1\}$ if $ a \equiv 0 \mod(2)$ and $= \{1, p^{\frac{a+1}{2}} \}, , a \equiv 1 \mod(2)$

  3. Follows from 1) and 2):

$$C(n) = \prod_{p|n, v_p(n) \equiv 1 \mod(2)}C(p^{v_p(n)})=\prod_{p|n, v_p(n) \equiv 1 \mod(2)} \{ 1, p^{\frac{v_p(n)+1}{2}} \}$$

  1. Follows from 3):

$$C(n)=\{ \sqrt{d \operatorname{rad}(d)}: d|n, \gcd(d,n/d)=1, \forall p| d : v_p(d) \equiv 1 \mod(2) \}$$

  1. Follows from 4): For an odd perfect number in Euler form $n = q^a m^2, q \equiv a \equiv 1 \mod(4),q$ prime, it follows that $C(n)=\{1,q^{\frac{a+1}{2}}\}$

Odd perfect numbers:

Let now $n$ denote an odd perfect number and $e:=q^{\frac{a+1}{2}}$ and consider $\chi_e = \bar{\chi}_e$. With this character, we can divide the set $D_n$ into two subsets:

$$D_+(n) \cup D_-(n) = D_n$$

defined through: $D_\pm(n):= \{d \in D_n| \chi_e(d) = \pm 1\}$. The interesting part is now that there are two bijections on these sets given by:

$$\alpha: D_+(n) \rightarrow D_-(n) : d^+ \mapsto \frac{n}{d^+}$$

and

$$\beta: D_+(n) \rightarrow D_-(n) : d^+ \mapsto q d^+$$

Meta-Question: Can there be said anything new relating odd perfect number using this character and these two bijections? (I have done some calculations along these lines and wanted to see if there are some other ideas relating these issues.)

Are there other usages of abelian characters to odd perfect numbers?

Thanks for your help!

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  • $\begingroup$ Question: Are you using anything about n being an odd perfect number other than Euler's theorem? Since Euler's theorem applies not just to odd perfect numbers but to any odd $n$ where $\sigma(n) \equiv 2$ (mod $4$), one needs at least some other ingredient here to be getting something nontrivial out of this. $\endgroup$
    – JoshuaZ
    Commented Nov 9, 2023 at 21:14
  • $\begingroup$ @JoshuaZ: I am using of course the definition of perfect number $\sigma(n)= 2n$ and the definition of $\chi_e$ $\endgroup$ Commented Nov 10, 2023 at 2:51
  • $\begingroup$ @JoshuaZ: Related sequence in OEIS, which is still in review: oeis.org/history/view?seq=A367197&v=3 . If $n$ is an odd perfect number in Euler form $n=q^a m^2$ then $n = ((q+1)/2) \cdot (tr(n)/2) $ where $tr(n)$ is defined in the OEIS sequence and it is the trace of the addition matrix of the divisors of $n$. $\endgroup$ Commented Nov 10, 2023 at 8:14
  • $\begingroup$ where are you using $\sigma(n)=2n$ rather than just using that $\sigma(n) \equiv 2$ (mod 4)? $\endgroup$
    – JoshuaZ
    Commented Nov 10, 2023 at 13:44
  • $\begingroup$ @JoshuaZ: I meant to say: I have used it in my computations, which I have not detailed here. $\endgroup$ Commented Nov 10, 2023 at 16:22

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