In Manjul Bhargava's The Factorial Function and Generalizations he motivates a new type of factorial $n!_S$ using by generalizing three theorems:
For $k, l \in \mathbb{Z}$, we have $k! \times l!$ divides $(k+l)!$.
For any primitive polynomial $f(x) \in \mathbb{Z}[x]$ with $\deg f = k$ then $\mathrm{gcd}\{ f(a): a \in \mathbb{Z}\}$ divides $k!$
In the process of solving generalizing these two results, he invents a factorial for any set of integers $S \in \mathbb{Z}$. For any prime $p$, order the element of $S$ by:
- choose $a_0 \in S$
- find $a_1$ giving the smallest power of $(a_1 - a_0)$
- find $a_2$ giving the smallest power of $(a_2 - a_0)(a_2 - a_1)$
- ...
- find $a_k$ giving the smallest power of $\prod_{i< k} (a_k - a_i)$
Here is my question:
- Bhargava observes that the integers in order $1,2,3, \dots, n$ form a $p$-ordered sequence for all $p$.
- Later in the same paper he observes the squares do as well $1^2,2^2,3^2, \dots, n^2$
- He also observes the powers of $2$ form a simultaneous $p$-ordering $1,2,4,8,\dots$
Does there exist a characterization of $S$ for when such a simultaneous ordering exists for all $p$? How can we read off the primes for which this isn't the case?
To my knowledge, this question remains open. It is difficult to characterize such sets. I would point out that all Fermat numbers is one of those sets [1]. More generally, if $E = O_f (a)$ is an the orbit of an integer $a$ under the iterated action of a polynomial $f \in \mathbb{Z}[X]$, then the sequence is simultaneous ordering exists for all $p$ (for sequence $E$).
Y fares