Is this divisor-sum camouflage criterion for prime-order Dirichlet characters correct, and is it already known?
问题内容
Let \chi be a Dirichlet character of prime order r, and define
[ A_\chi(n)=\sum_{d\mid n}\chi(d). ]
For a prime p, one trivially has
[ A_\chi(p)=1+\chi(p). ]
I have been looking at composites n that satisfy the same identity
[ A_\chi(n)=1+\chi(n). \tag{1} ]
I would like to know whether the following argument is correct, whether I am overlooking a standard theorem, and in particular whether the constructive converse below has a gap.
Assume throughout that n is unramified for \chi.
Write
[ n=\prod_{i=1}^s p_i^{e_i}, ]
and choose a primitive r-th root of unity \zeta_r. Write
[ \chi(p_i)=\zeta_r^{a_i}. ]
Then
[ A_\chi(n)
\prod_i \left( 1+\zeta_r^{a_i} +\zeta_r^{2a_i} +\cdots+ \zeta_r^{e_i a_i} \right). ]
Define the integer polynomial
[ F(x)= \prod_i \left( 1+x^{a_i}+\cdots+x^{e_i a_i} \right)
1-x^{\sum_i e_i a_i}. ]
If (1) holds, then F(\zeta_r)=0. Since r is prime, the minimal polynomial of \zeta_r is
[ \Phi_r(x)=1+x+\cdots+x^{r-1}. ]
Thus \Phi_r(x)\mid F(x). Evaluating at x=1 gives
[ r=\Phi_r(1)\mid F(1)
\prod_i(e_i+1)-2. ]
Therefore
[ \boxed{\tau(n)\equiv2\pmod r.} \tag{2} ]
So (2) is a necessary condition for a composite to satisfy the prime-like identity (1).
There also seems to be a constructive converse at the level of character states.
Set
[ m_i=e_i+1 ]
and define mixed-radix weights
[ W_1=1,\qquad W_i=\prod_{h<i}m_h. ]
Every divisor
[ d=\prod_i p_i^{j_i}, \qquad 0\le j_i<m_i, ]
corresponds uniquely to
[ t=\sum_i j_iW_i \in{0,1,\ldots,\tau(n)-1}. ]
Now prescribe
[ \chi(p_i)=\zeta_r^{W_i}. \tag{3} ]
Then
[ \chi(d)=\zeta_r^t, ]
and therefore
[ A_\chi(n)
\sum_{t=0}^{\tau(n)-1}\zeta_r^t. ]
If
[ \tau(n)=kr+2, ]
then
[ A_\chi(n)
k(1+\zeta_r+\cdots+\zeta_r^{r-1}) +1+\zeta_r
1+\zeta_r. ]
Also,
[ \sum_i e_iW_i=\tau(n)-1, ]
so
[ \chi(n)=\zeta_r^{\tau(n)-1}=\zeta_r. ]
Hence
[ A_\chi(n)=1+\chi(n). ]
Thus the congruence
[ \tau(n)\equiv2\pmod r ]
appears to be not only necessary, but constructively sharp at the character-state level.
For several compatible characters \chi_j of distinct prime orders r_j, the same mixed-radix weights can be used in every channel:
[ \chi_j(p_i)=\zeta_{r_j}^{W_i}. ]
This suggests the simultaneous necessary condition
[ \boxed{ \tau(n)\equiv2 \pmod{\operatorname{lcm}(r_1,\ldots,r_k)} } ]
is likewise constructively sharp, provided the prescribed joint character values are actually realizable by primes. I am assuming here that the chosen characters are independent enough that the joint character map onto the desired roots-of-unity states is surjective; CRT and Dirichlet's theorem would then supply primes in the corresponding reduced residue classes.
As one consequence, if n is squarefree with s=\omega(n), then
[ \tau(n)=2^s. ]
For character orders 11,23,41, simultaneous camouflage requires
[ 2^{s-1}\equiv1 \pmod{11,23,41}. ]
Since
[ \operatorname{ord}{11}(2)=10,\qquad \operatorname{ord}{23}(2)=11,\qquad \operatorname{ord}_{41}(2)=20, ]
one obtains
[ 220\mid(s-1). ]
Thus, apart from the prime case s=1, the first squarefree value not excluded by this condition is
[ s=221. ]
My questions are:
Is the cyclotomic argument proving [ A_\chi(n)=1+\chi(n)\Rightarrow\tau(n)\equiv2\pmod r ] correct as stated?
Is the mixed-radix construction a valid converse at the character-state level?
Under the stated independence/surjectivity condition on several characters, is the passage from prescribed character states to actual prime factors justified as I have described it?
Is this criterion or mixed-radix construction already standard under another name or contained in known results on twisted divisor sums, cyclotomic polynomials, or Dirichlet characters?
I am not claiming this gives a faster primality test or factoring algorithm. My interest is the structural question of which composite factorizations can imitate the identity A_\chi(p)=1+\chi(p) simultaneously across several character channels.
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