# Calculate the limit $\lim \limits_{n \to \infty} |\sin(\pi \sqrt{n^2+n+1})|$

Calculate

$$\lim \limits_{n \to \infty} |\sin(\pi \sqrt{n^2+n+1})|$$

-
Is $n$ here required to be an integer, or a real number? – MJD Apr 2 '13 at 22:10
As MJD hints, if n is a real number, then your limit doesn't exist, since you're bouncing between 0 and 1. – Calvin Lin Apr 2 '13 at 22:14
The answers all take $n$ to be restricted to the integers, their answers would equally apply if $n$ was restricted to the naturals as well, right? – user66807 Apr 23 '13 at 0:20

Note that $$\sqrt{n^2+n+1}-n=\frac{n+1}{\sqrt{n^2+n+1}+n}\to 1/2$$ as $n\to\infty$.

For even $n$, $\sin(\sqrt{n^2+n+1}\pi)=\sin(\sqrt{n^2+n+1}\pi-n\pi)\to \sin(\pi/2)=1$ as $n\to\infty, n$ even.

For odd $n$, $\sin(\sqrt{n^2+n+1}\pi)=-\sin(\sqrt{n^2+n+1}\pi-n\pi)\to -\sin(\pi/2)=-1$ as $n\to\infty, n$ odd.

Therefore, $$|\sin(\sqrt{n^2+n+1}\pi)|\to 1$$ as $n\to\infty$.

-

Edit: This assumes that $n$ is restricted to the integers.

Firstly, since $\mid \sin \theta \mid$ is periodic with period $\pi$, we want to look at the value of $\pi \sqrt{n^2 + n +1} \pmod{\pi}$.

Secondly, convince yourself that $\lim_{n\rightarrow \infty} \pi \sqrt{n^2+n+1} \pmod{\pi} = \frac {1}{2} \pi$.

Thirdly, by the continuity of $\sin \theta$, $\mid \lim_{n \rightarrow \infty} \sin (\pi \sqrt{n^2+n+1} ) \mid = \mid \sin \lim_{n\rightarrow \infty} \pi \sqrt{n^2+n+1}\mid = \sin \frac {1}{2} \pi$.

-
Can you help us convince ourselves? – levitopher Apr 2 '13 at 22:16
So, it's oscillating around $\pm1$, no? – Berci Apr 2 '13 at 22:17
Because of the multiplying by $n$, to the front of the square root. You should be approximating your square root by $1 + \frac {1}{2n}$ instead. Moreover, note that $lim_{n\rightarrow \infty} n f(n) \neq n \lim_{n \rightarrow \infty} f(n)$. – Calvin Lin Apr 2 '13 at 22:24

The function $|\sin x|$ is periodic with period $\pi$. Hence

$$\lim \limits_{n \to \infty}|\sin \pi \sqrt{n^2+n+1}|= \lim \limits_{n \to \infty}|\sin \pi (\sqrt{n^2+n+1}-n)|$$

But

$$\lim \limits_{n \to \infty} (\sqrt{n^2+n+1}-n)=\lim \limits_{n \to \infty} \dfrac{n^2+n+1-n^2}{\sqrt{n^2+n+1}+n}=\dfrac{1}{2}$$

It follows that the limit we are computing is equal to $\left |\sin \dfrac{\pi}{2}\right |$, which is 1.

-
Yes, that's a neat elementary way. But don't avoid Taylor too long...One day you'll have no choice! – 1015 Apr 2 '13 at 22:35

If you know a little bit about Taylor expansions and big O notation, you can do that easily. We will use $$\sqrt{1+u}=1+\frac{u}{2}+O(u^2)\qquad \mbox{when } u\longrightarrow 0.$$ Now $$\pi\sqrt{n^2+n+1}=\pi n\sqrt{1+\frac{1}{n}+\frac{1}{n^2}}=\pi n\left(1+\frac{1}{2n}+O\left(\frac{1}{n^2} \right)\right)=\pi n+\frac{\pi}{2}+O\left(\frac{1}{n} \right)$$ So $$\sin (\pi\sqrt{n^2+n+1})=\sin\left(\pi n+\frac{\pi}{2}+O\left(\frac{1}{n} \right) \right)=(-1)^n\sin\left(\frac{\pi}{2}+O\left(\frac{1}{n} \right) \right).$$ It follows that $$|\sin (\pi\sqrt{n^2+n+1})|=\left| \sin\left(\frac{\pi}{2}+O\left(\frac{1}{n} \right) \right)\right|\longrightarrow \left| \sin\left(\frac{\pi}{2}\right)\right|=1.$$

-

We have $$\sqrt{n^2+n+1}=n\sqrt{1+\frac{1}{n}+\frac{1}{n^2}}=n\left(1+\frac{1}{2n}+o(\frac{1}{n})\right)=n+\frac{1}{2}+o(1)$$ hence $$\lim \limits_{n \to \infty} |\sin(\pi \sqrt{n^2+n+1})|=\lim \limits_{n \to \infty}|\sin(n\pi+\frac{\pi}{2}+o(1))|=\lim \limits_{n \to \infty}|\cos(o(1))|=1.$$

-