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Is it true that for integers $n > e, x \ge n$, it is impossible for $\dfrac{(x+n)!}{x!}$ to be divisible by $n$ distinct primes where each prime is greater than $\dfrac{(x+n)e}{n}$?

Example:

  • $x=n=10$

There cannot be $10$ primes greater than $\dfrac{20}{e}{10} \approx 5.44$ that divide $$\dfrac{20!}{10!} = 11\times 12\times 13\times 14\times 15\times 16\times 17\times 18\times 19\times 20$$

It is not possible for $7^{10}$ or any combination of $10$ primes where each prime is greater or equal to $7$ to divide $\dfrac{20!}{10!}$

Here's my thinking:

(1) Let $n>1$ and $x \ge n$ be integers.

(2) If a prime $p>n$ divides $\dfrac{(x+n)!}{x!}$, then it necessarily divides ${{x+n}\choose{n}}$ since it is not divisible by $n!$.

(3) From a well known inequality:

$${{x+n}\choose{n}} < \left(\frac{(x+n)e}{n}\right)^n$$

(4) If all $n$ primes are greater than $\dfrac{(x+n)e}{n}$, then it follows:

$$p_1{p_2}\dots{p_n} > \left(\frac{(x+n)e}{n}\right)^n > {{x+n}\choose{n}}$$

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  • $\begingroup$ I think that there is an error in your thinking. In (2), you are considering $p$ such that $p\gt n$. However, in (4), you have $p_i\gt\frac{(x+n)e}{n}$ which does not imply $p_i\gt n$. $\endgroup$
    – mathlove
    Commented May 25, 2020 at 14:35
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    $\begingroup$ I think that one can prove in your way that it is true at least when $n\gt e$ and $x\ge\frac{n(n-e)}{e}$. $\endgroup$
    – mathlove
    Commented May 25, 2020 at 14:44

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