Show that $n \log(n!) = n \log n − n + \mathcal{O}(\log n).$

Question

I am stuck at the following exercise:

Show that for $n \rightarrow \infty$ holds

$$\log(n!) = n \log n − n + \mathcal{O}(\log n).$$

I do not see how I should prove this. I know that

$$\log(n!) = \log(n)+\log(n-1)+\cdots+\log(1)$$

, and I see that this is similar to $$n\log(n) = \sum_{i=1}^n \log(n)$$but I do not see how this should help here. Could you give me a hint?

Answer 1

Let us treat this as an exercise in summation by parts:

$$ \log(N!)=\sum_{n=1}^{N}\log(n)=N\log N-\sum_{n=1}^{N-1}n\log\left(1+\frac{1}{n}\right) $$ and by Maclaurin series $\log(1+x)=x-\frac{x^2}{2}+O(x^3)$, so $$ \log(N!)=N\log N-(N-1)+\sum_{n=1}^{N-1}n\left(\frac{1}{n}-\log\left(1+\frac{1}{n}\right)\right)$$ equals $$ N\log N-N+1+\sum_{n=1}^{N-1}\left(\frac{1}{2n}+O\left(\frac{1}{n^2}\right)\right) $$ i.e. $$ N\log N - N + \frac{1}{2}\log N + O(1). $$ Probably the hardest part of the proof of Stirling's inequality is to show that the last $O(1)$ term hides $\frac{1}{2}\log(2\pi)+O\left(\frac{1}{n}\right)$, but this weaker form is usually more than enough for concrete applications.

Answer 2

$$\int_{1}^{n} \log x dx < \log n! = \sum_{k=1}^{n} \log k < \int_{1}^{n+1}\log x dx $$ Can you solve for these integrals?

Answer 3

$\sum_{i=1}^n \log(i)$ is a left hand Riemann sum for $\int_1^{n+1} \log(x) dx$. It is also a right hand sum for $\int_0^n \log(x) dx$.

Now $\log$ is an increasing function, so its right hand sums are bigger than its integrals and its left hand sums are smaller than its integrals, so your sum lies between these two numbers, and the difference between these two numbers turns out to be $O(\log(n))$ as $n \to \infty$ (which you should show yourself).

One can make one of the bounds slightly better by noticing that $\sum_{i=1}^n \log(i) = \sum_{i=2}^n \log(i)$ since $\log(1)=0$. This sum starting at $2$ is now a right hand sum for $\int_1^n \log(x) dx$. For this particular exercise, either way will suffice.