# Upper bound for falling factorial

Show that the falling factorial $(n)_k=\dfrac{n!}{(n-k)!}$ satisfies the following inequality: $$(n)_k\le n^ke^{\frac{-k(k-1)}{2n}}$$

I rewrote the inequality as $\left(1+\frac{1}{n-1}\right)\left(1+\frac{2}{n-2}\right)\ldots\left(1+\frac{k-1}{n-(k-1)}\right)\ge e^{\frac{k(k-1)}{2n}}$.

Then I tried to apply directly the inequality $1+x\ge e^{x-\frac{x^2}{2}}$, for $x\ge 0$.

Then it suffices to show $\displaystyle \sum^{k-1}_{i=1} \left(\frac{i}{n-i}-\frac{i}{n}-\frac{i^2}{2(n-i)^2}\right)\ge 0\iff \sum^{k-1}_{i=1} \left(\frac{i^2}{n(n-i)}-\frac{i^2}{2(n-i)^2}\right)\ge 0$

However this is only good enough for $k<\frac{n}{2}$. Any way from here?

An inequality $1+x\le\exp(x)$ which holds for all real $x$ implies the following chain

$$\frac {(n)_k}{n^k}=\prod_{i=0}^{k-1} \left (1-\frac{i}{n}\right) \le \prod_{i=0}^{k-1} \exp\left (-\frac{i}{n}\right)= \exp\left(\sum_{i=0}^{k-1}-\frac{i}{n}\right)= \exp\left(-\frac{(k-1)k}{2n}\right).$$