<s>[INST] Show that $\prod_{k=0}^n |x-k| \le (n-1)!/2$ for $1 \le x \le n-1$. 
Let $n \ge 3$, $x \in \Bbb R$ such that $1 \le x \le n-1$. Show that $\prod_{k=0}^n |x-k| \le (n-1)!/2$.

For $n=3$, $1 \le x \le 2$ we want to show $|x(x-1)(x-2)(x-3)|=x(x-1)(2-x)(3-x) \le 1$.
We see that some large bounds are easy to guess:
$$
x(x-1)(2-x)(3-x) \le 2 \times 1 \times 1 \times 2=4
$$
but not precise enough. Another try:
$$
x(x-1)(2-x)(3-x) \le x(3-x) \le 9/4
$$
the last inequality is from the study of $f(x)=x(3-x)$.
Same problem.
For $n=3$ and $1 \le x \le 2$ let $0 \le y=x-1 \le 1$.
We get using AM-GM
\begin{align}
|x(x-1)(x-2)(x-3)| &= |(y+1)(y)(y-1)(y-2)|\\
&=y(1-y^2)(2-y)\\
&\le (\frac{3-y^2}{3})^3\\
&\le  1
\end{align}
For the general case I tried to use the same approach:
let $1 \le x \le n-1$ ie. $0 \le y = (x-1)/(n-2) \le 1$. We have
\begin{align}
\prod_{k=0}^n |x-k|
&=
\prod_{k=0}^n |(n-1)y +1-k|\\
&=
\prod_{k=0}^l |(n-1)y +1-k| \prod_{k=l+1}^n |(n-1)y +1-k|\\
&=
\prod_{k=0}^l ((n-1)y +1-k) \prod_{k=l+1}^n (k-1-(n-1)y)\\
\end{align}
where $l = \lfloor (n-1)y+1\rfloor$. Applying AM-GM doesn't really help here. Suppose $y<1$ ie. $l<n$, we see that
$$
\prod_{k=0}^l ((n-1)y +1-k) \le \prod_{k=0}^l (n-k)=\frac{n!}{(n-l-1)!}
$$
and
$$
\prod_{k=l+1}^n (k-1-(n-1)y) \le \prod_{k=l}^{n-1} (k) = \frac{(n-1)!}{(l-1)!}
$$
So
$$
\prod_{k=0}^n |x-k|\le
\frac{(n-1)!}{(l-1)!}\frac{n!}{(n-l-1)!}
$$
The bound is too big...
I also tried this approach: let $f(x)= \prod_{k=0}^n (x-k)$, $f'(x)=f(x)(\sum_{k=0}^n \frac{1}{x-k})$.
I'm trying to find informations about $x$ such that $f'(x)=0$. Let $x \in \Bbb R - [|0,n|]$. So $\sum_{k=0}^n \frac{1}{x-k}=0$. But I'm stuck here.
My questions are:

*

*Does someone have a hint or a proof?

*Can someone give me some advice on how to handle such problems during an exam?

Thanks :)
 [/INST] I would like to solve a more general problem than this problem and that problem.
First, we find all local maxima of the function $f(x) =\prod_{k=0}^n |x-k|$ in the interval $0 \le x \le n$.
For each interval $[p,p+1)$ with $p\in \{0,...,n-1\}$ we prove that there exists 1 local maximum.
We guess the value of $v_0,v_1,...,v_n$ such that there exists $x_p \in [p,p+1)$ satisfying two following conditions
$$v_0x = v_1(x-1) =...=v_n(x-k) \tag{1}$$
$$v_0+v_1+...+v_n=0\tag{2}$$
Solve these 2 conditions, we can have a solution (attention, this is not a unique solution, but we need only 1) as follows
$$v_k=\frac{1}{x_p-k}$$
with $x_p \in [p,p+1)$ satisfying this equation
$$\sum_{0 \le k \le n}\frac{1}{x_p-k} = 0 \tag{3}$$
Note: For information, it's easy to prove that the equation (3) has exactly one root in each interval $[p,p+1)$ where $p\in \{0,...,n-1\}$.
Now, we apply the AM-GM inequality for $v_k(x-k)$ with $0 \le k \le n$ (we notice that for $k >p$, $v_k$ is negative, then the terms $v_k(x-k)$ are all positive).
\begin{align}
f(x) &=\prod_{0 \le k \le n}|x-k| \\
 &= \frac{1}{\prod_{0 \le k \le n}|v_k|} \prod_{0 \le k \le n}v_k(x-k) \\
 &\le \frac{1}{\prod_{0 \le k \le n}|v_k|} \left(\frac{\sum_{0 \le k \le n}v_k(x-k)}{n+1}   \right)^{n+1} =  \frac{1}{\prod_{0 \le k \le n}|v_k|k} \left(\frac{-\sum_{0 \le k \le n}kv_k}{n+1}   \right)^{n+1}
\end{align}
We note that,;from (1) and (3), we have
\begin{align}
-\sum_{0 \le k \le n}kv_k &=  -\sum_{0 \le k \le n}\frac{k}{x_p-k} \\
&= n+1 - x_p\sum_{0 \le k \le n}\frac{1}{x_p-k} \\
&= n+1
\end{align}
Hence,
$$f(x) \le \frac{1}{\prod_{0 \le k \le n}|v_k|} \left(\frac{-\sum_{0 \le k \le n}kv_k}{n+1}   \right)^{n+1} = \prod_{0 \le k \le n}|x_p-k|\left(\frac{n+1}{n+1}   \right)^{n+1} = \prod_{0 \le k \le n}|x_p-k|$$
The local maximum in $x \in [p,p+1)$ occurs when $x = x_p$ where $x_p \in [p,p+1)$ is the solution of (3).
Second, we prove that $f(x_0) \ge f(x_1) \ge ...\ge f(x_{[\frac{n}{2}]})$ and $f(x)$ is a symmetric function at $x = \frac{n}{2}$ (in other words, $f(x) = f(n-x)$).
It's easy to prove the second statement about the symmetry.
For the first statement, for $x \in [p,p+1)$ with $p\in \{0,...,[\frac{n}{2}]-1\}$, we have
$$ f(x+1) = \prod_{0 \le k \le n}|(x+1)-k| = \frac{x+1}{|x-n|}\prod_{0 \le k \le n}|x-k| = \frac{x+1}{|x-n|} f(x)$$
As $x \in [p,p+1)$ with $p\in [0,...,([\frac{n}{2}]-1)]$, we have then $x< \frac{n+1}{2}$, so $\frac{x+1}{|x-n|} <1$ or $f(x+1) < f(x)$. We deduce that
$$\max_{x \in [p+1,p+2)}f(x) = \max_{x \in [p,p+1)}f(x+1) < \max_{x \in [p,p+1)}f(x)$$
or for all $p\in \{0,...,[\frac{n}{2}]-1\}$ $$f(x_{p})>f(x_{p+1}) \tag{4}$$
Finally, we solve your problem, which corresponds to the case: find the maximum of $f(x)$ in the interval $ 1 \le x \le n-1$  (another particular case is where $0 \le x \le n$ was solved here ).
From (4), we have the function $f(x)$ attains its maximum value $f(x_1)$ in $x \in [1,n-1]$ when $x=x_1$ where $x_1 \in (1,2)$ is the root of the equation (3).
$$f(x) \le f(x_1) = \prod_{0 \le k \le n}|x_1-k| = x_1(x_1-1)(2-x_1) \prod_{3 \le k \le n}(k-x_1) \le \left(\frac{x_1 + (x_1-1) + 2(2-x_1)}{3}  \right)^3 \frac{1}{2}\prod_{3 \le k \le n}(k-1) = \frac{(n-1)!}{2}$$
Because $x_1$ doesn't satisfy the condtion of AM-GM inequality ($x_1 = x_1-1 = 2(2-x_1)$), we conclude that $$f(x) < \frac{(n-1)!}{2}$$
</s>