# Least Common Multiple and the product of a sequence of consecutive integers

Let $$x>0, n>0$$ be integers.

Let LCM$$(x+1, x+2, \dots, x+n)$$ be the least common multiple of $$x+1, x+2, \dots, x+n$$.

Let $$v_p(u)$$ be the highest power of $$p$$ that divides $$u$$.

It seems to me that:

$$\frac{(x+1)(x+2)\times\dots\times(x+n)}{\text{LCM}(x+1, x+2, \dots, x+n)} \le (n-1)!$$

Here is my thinking:

• Let $$p$$ be any prime that divides $$(x+1)\times\dots\times(x+n)$$.

• Let $$x+i$$ be the integer in $$\{x+1,x+2. \dots, x+n\}$$ that is divisible by the highest power of $$p$$.

• All integers in the sequence $$\{x+1, x+2, x+3, \dots, x+n\}$$ that are divisible by $$p$$ will have the form $$x + i \pm ap$$ where $$0 < a \le \left\lfloor\dfrac{n-1}{p}\right\rfloor$$.

• $$v_p\left(\dfrac{(x+1)(x+2)\times\dots\times(x+n)}{\text{LCM}(x+1,x+2,\dots,x+n)}\right) \le v_p(p) + v_p(2p) + \dots + v_p\left(\left\lfloor\dfrac{n-1}{p}\right\rfloor p\right) = v_p((n-1)!)$$

• So, it follows that the maximum power of $$p$$ that divides $$\dfrac{(x+1)\times\dots\times(x+n)}{\text{LCM}(x+1,\dots,x+n)}$$ is less than or equal to the maximum power of $$p$$ that divides $$(n-1)!$$

Edit: Made changes to third and fourth bullet point with an attempt to make them cleaner.

Edit 2: I originally put the "same" power in the last bullet point. That is not correct. It should be less or equal. I have changed it.

Edit 3: Fixed a typo. n! should be (n-1)!.

• Are the third and fourth parts in your thinking saying that $v_p((x+1)\cdots (x+n))\le v_p(x+i)+\lfloor\frac{n-1}{p}\rfloor$ ? If not, can you make it clearer? – mathlove Sep 20 '18 at 7:13
• @mathlove, thanks for the feedback. I have cleaned up the text in the question. I hope that my thinking is now clearer. – Larry Freeman Sep 21 '18 at 15:03
• OK, I'll read them carefully. By the way, the last bullet point is not true in general. Take $x=1,n=9,p=2$. – mathlove Sep 21 '18 at 15:24
• Very good point. The maximum power is less or equal not necessarily the same. I will update the argument. – Larry Freeman Sep 21 '18 at 15:46
• Interpreted literally, the third bullet point is not ture. Take $x=3,n=3,p=2$. Then, $x+i-p$ is not in $\{x+1,\cdots, x+n\}$. Also, in the fourth bullet point, the equation $v_p(p) + v_p(2p) + \dots + v_p\left(\left\lfloor\dfrac{n-1}{p}\right\rfloor p\right) = v_p(n!)$ is not true. Take $n=8,x=1,p=2$. – mathlove Sep 22 '18 at 4:09

Changing $$n,k$$ to $$x+n,n-1$$ respectively in$$\binom nk\le\frac{\text{LCM}(n-k,n-k+1,\cdots, n)}{n-k}$$ we get $$\binom{x+n}{n-1}\le\frac{\text{LCM}(x+1,x+2,\cdots, x+n)}{x+1}$$ which is your claim.