# Justify: if $x\gt 0$, $\;\lim_{n\to\infty} \sqrt{n}\cdot{\overbrace{\sin\sin\cdots\sin}^{n\space\text{sines}}(x)}=\sqrt{3}$

I believe that I have managed to show that (if $x\gt 0$) $$\lim_{n\to\infty} \sqrt{n}\cdot{\overbrace{\sin\sin\cdots\sin}^{n\space\text{sines}}(x)}=\sqrt{3}$$ I did this by defining a sequence as $a_0=x$ and the recursion $$a_{n+1}=\sin a_n$$ I then approximated the recursion with the first two nonzero terms of the Maclaurin series for sine, giving me $$\Delta a_n=-\frac{x^3}{6}$$ I then approximated this with a differential equation $$y'=-\frac{y^3}{6}$$ Which I then easily solved... the answer follows from here.

Question: How can this be made more rigorous? I don't know how to justify that my approximations are good enough for th error to vanish under the limit. What theorems are generally used to justify approximations of discrete recursions with differential equations? I think I know how to justify the approximation of some with its Maclaurin series using the Lagrange error bound.

• @Dzoooks It is a finite difference. $$\Delta a_n:=a_{n+1}-a_n$$ – Franklin Pezzuti Dyer Jul 30 '18 at 15:48
• How is this possible? $$|\overbrace{\sin \dots \sin}^{n \ \text{sines}}(x)| \leq 1,$$ so your limit is 0 by the Squeeze Theorem. Should you be dividing by $\sqrt{x}$? – Dzoooks Jul 30 '18 at 15:51
• This is true for $x \in (0, \pi/2): \lim a_n\sqrt n = \sqrt 3$, which could be justified by applying Stolz formula. – xbh Jul 30 '18 at 15:53
• @xbh and Dzoooks: sorry, thanks for pointing that out. The $\sqrt n$ should be multiplied. – Franklin Pezzuti Dyer Jul 30 '18 at 15:56
• @xbh Can you explain why this only works for x in that interval? And if you walked me through the application of Stolz and how to carefully justify this, that would be the kind of answer I am looking for. In particular, I would like to justify the fact that the error is small enough to be negligible as n goes to infinity. – Franklin Pezzuti Dyer Jul 30 '18 at 16:00

## 4 Answers

This may not be the answer you wanted, cuz it may not be generalized.

Claim: $\lim n a_n^2 = 3$.

Proof. $\blacktriangleleft$ Note that not matter which value $x$ is, $a_1 = \sin (x) \in (-1,1)$, so it suffice to consider if $x \in (0, \pi/2)$. Now $a_1 > 0$ and $a_2 < a_1$. By induction it would be easy to verify that $0 < a_n< a_{n-1}$. Hence $a_n \searrow$ and $a_n > 0$. Therefore the limit $A = \lim a_n$ exists. Solve $\sin (A) = A$ in $[0, \pi/2]$ gives that $A=0$, i.e. $a_n = o(1)$.

Now compute the limit: \begin{align*} \lim n a_n^2 &= \lim \frac n {a_n^{-2}} \\ &= \lim \frac 1 {a_{n+1}^{-2} - a_n^{-2}}\quad [\text {Stolz formula}]\\ &= \lim \frac {a_n^2 a_{n+1}^2} {a_n^2 - a_{n+1}^2} \\ &= \lim \frac {a_n^2 \sin(a_n^2)} {(a_n - \sin(a_n) )(a_n + \sin (a_n))} \\ &= \lim \frac {a_n^4} {a_n^3/6 \cdot 2a_n} \quad [\sin(a_n) \sim a_n; a_n - \sin (a_n) \sim a_n^3/6]\\ &= 3. \end{align*} Hence $\lim a_n \sqrt n = \sqrt 3. \blacktriangleright$

• Even if you start with $x \ge \pi/2$, already the next iterate $\sin x$ is in the interval $[0,\pi/2)$, so there's no problem. – Hans Lundmark Jul 30 '18 at 18:15
• @HansLundmark Thanks, I forgot about this. – xbh Jul 30 '18 at 18:30
• Very clever use of Cesaro-Stolz. +1 – Paramanand Singh Jul 31 '18 at 0:43
• @ParamanandSingh Thanks! – xbh Jul 31 '18 at 2:07

I'll assume $0<x<1$, so that your sequence decreases to zero.

Write $$b_n=a_n^2=\sin^2(\sin^{[n-1]}(x))$$ where I'll use $\sin^{[m]}(x)$ for the $m$-fold composite of $\sin$. We'd like to prove $b_n\sim 3/n$. But $b_{n+1}=\phi(b_n)$ where $$\phi(t)=\sin^2\sqrt t=t-\frac{t^2}3+\frac{t^3}{12}+\cdots.$$ Then $$\frac1{b_{n+1}}=\frac1{b_n}\left(1+b_n+O(b_n^2)\right) =\frac1{b_n}+\frac13+O(b_n).$$ As $b_n\to0$ then $1/b_n$ increases by at least $1/4$ eventually, so that $1/b_n<n/4+C$ for some constant $C$, and so $b_n=O(1/n)$. Then$$\frac1{b_n}=\frac n3+O(\ln n)$$ which is good enough.

• Better than mine. Thanks for the elegant analysis! – xbh Jul 30 '18 at 16:19
• This is hard to understand. How did you obtain the expression for $1/b_{n+1}$? – Franklin Pezzuti Dyer Jul 30 '18 at 16:47
• @Frpzzd By writing $1/b_{n+1}=1/\phi(b_n)$ and taking the first few terms of the Laurent series. – Angina Seng Jul 30 '18 at 16:49

We have

$$\sqrt n \sin(\cdots \sin x) = a_n\\ \sqrt{n+1}\sin(\sin(\cdots \sin x)) = a_{n+1}$$

hence

$$\sin\left(\frac{a_n}{\sqrt n}\right) = \frac{a_{n+1}}{\sqrt{n+1}}$$

Analyzing now the iterative procedure

$$u_{n+1} = \sin u_n$$

with $u_0 = \frac{\pi}{2}$ the sequence

$$\sqrt k u_k$$

is convergent and has a behavior shown in the attached plot

(in red $\sqrt 3$ and in blue $\sqrt k u_k$) Since you asked for a reference, here is one that I have enjoyed for many years:

Asymptotic Methods in Analysis by N. G. de Bruijn, available quite inexpensively at Dover and Amazon.

Chapter 8 has an extensive discussion of iterated functions which includes most of the methods in the answers to this question.

• Thank you very much! I will have to add this to my "to read" list. – Franklin Pezzuti Dyer Aug 2 '18 at 19:22