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I would like to investigate the convergence of

$$\sqrt{1+\sqrt{2+\sqrt{3+\sqrt{4+\sqrt\ldots}}}}$$

Or more precisely, let $$\begin{align} a_1 & = \sqrt 1\\ a_2 & = \sqrt{1+\sqrt2}\\ a_3 & = \sqrt{1+\sqrt{2+\sqrt 3}}\\ a_4 & = \sqrt{1+\sqrt{2+\sqrt{3+\sqrt 4}}}\\ &\vdots \end{align}$$

Easy computer calculations suggest that this sequence converges rapidly to the value 1.75793275661800453265, so I handed this number to the all-seeing Google, which produced:

Henceforth let us write $\sqrt{r_1 + \sqrt{r_2 + \sqrt{\cdots + \sqrt{r_n}}}}$ as $[r_1, r_2, \ldots r_n]$ for short, in the manner of continued fractions.

Obviously we have $$a_n= [1,2,\ldots n] \le \underbrace{[n, n,\ldots, n]}_n$$

but as the right-hand side grows without bound (It's $O(\sqrt n)$) this is unhelpful. I thought maybe to do something like:

$$a_{n^2}\le [1, \underbrace{4, 4, 4}_3, \underbrace{9, 9, 9, 9, 9}_5, \ldots, \underbrace{n^2,n^2,\ldots,n^2}_{2n-1}] $$

but I haven't been able to make it work.

I would like a proof that the limit $$\lim_{n\to\infty} a_n$$ exists. The methods I know are not getting me anywhere.

I originally planned to ask "and what the limit is", but OEIS says "No closed-form expression is known for this constant".

The references it cites are unavailable to me at present.

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    $\begingroup$ This is probably a good place to start: "It was discovered by T. Vijayaraghavan that the infinite radical $\sqrt{ a_1 + \sqrt{ a_2 + \sqrt{ a_3 + \sqrt{a_4 + \ldots }}}}$ where $a_n \ge 0$, will converge to a limit if and only if the limit of $\log a_n / 2^n$ exists" - Clawson, p. 229. (Taken from OEIS.) $\endgroup$ – George V. Williams Jul 6 '13 at 3:23
  • $\begingroup$ possible duplicate of Sum and Product of Infinite Radicals $\endgroup$ – MJD Jul 6 '13 at 3:26
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    $\begingroup$ I misread the solution at Sum and Product of Infinite Radicals. It asks several questions, one of which is mine, but all the answers provided are for the other questions. $\endgroup$ – MJD Jul 6 '13 at 3:28
  • $\begingroup$ Related: Nested radicals $\endgroup$ – MJD Jul 6 '13 at 3:31
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    $\begingroup$ This may help. $\endgroup$ – Maazul Jul 6 '13 at 4:44
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For any $n\ge4$, we have $\sqrt{2n} \le n-1$. Therefore \begin{align*} a_n &\le \sqrt{1+\sqrt{2+\sqrt{\ldots+\sqrt{(n-2)+\sqrt{(n-1) + \sqrt{2n}}}}}}\\ &\le \sqrt{1+\sqrt{2+\sqrt{\ldots+\sqrt{(n-2)+\sqrt{2(n-1)}}}}}\\ &\le\ldots\\ &\le \sqrt{1+\sqrt{2+\sqrt{3+\sqrt{2(4)}}}}. \end{align*} Hence $\{a_n\}$ is a monotonic increasing sequence that is bounded above.

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  • $\begingroup$ The first part following from $n^2 -4n +1 >0$ for $n\ge 4$. $\endgroup$ – Tom Collinge Jun 6 '14 at 14:17
  • $\begingroup$ Very elegant estimate! +1 $\endgroup$ – ZFR Nov 15 '15 at 17:06
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The first number $1$ is a nuisance, so at first we disregard it.

We proceed by induction, and deal with finite nested radicals that start with $\sqrt{k+\sqrt{(k+1)+\cdots}}$, where $k\ge 2$.

We will show that such a radical is $\lt 2k$, by induction on depth. The result is certainly true for all nested radicals of depth $1$, since $\sqrt{q}\lt 2q$.

For the induction step, a nested radical of depth $n$ that starts with $k$ is $\sqrt{k+R}$, where $R$ is a nested radical of depth $n-1$ that starts with $k+1$. By the induction assumption, we have $R\lt 2k+2$. But then $\sqrt{k+R}\lt \sqrt{3k+2}\lt 2k$ if $k \ge 2$.

So (finite) nested radicals of any depth that start with $2$ are $\lt 4$. The sequence of nested radicals is clearly increasing, so it converges. It follows that the nested radical of the post is $\le \sqrt{1+4}$.

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An easy sloppy way to see it: You know that $\sqrt{1+\sqrt{1+\sqrt{1+\sqrt{1+\dots}}}}=\phi$. Now multiply by $2$ to get $2\phi=2\sqrt{1+\sqrt{1+\sqrt{1+\sqrt{1+\dots}}}}=\sqrt{4+4\sqrt{1+\sqrt{1+\sqrt{1+\dots}}}}=\sqrt{4+\sqrt{16+\sqrt{256+\sqrt{256^2+\dots}}}}>\sqrt{1+\sqrt{2+\sqrt{3+\sqrt{4+\dots}}}}$. Of course this should be done more rigorous.

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If $( a_k )_{k\in\mathbb{N}}$ is any sequences of positive numbers such that:

$$0 \le a_k \le \alpha \lambda^{2^k}\quad\text{ for some }\quad \alpha, \lambda \in \mathbb{R}_{+}$$

Using same convention $\;[r_1,r_2\ldots] = \sqrt{r_1 + \sqrt{r_2 + \ldots}}\;$ as in the question, we have:

$$\begin{align} [a_n] & \le \sqrt{\alpha \lambda^{2^n}} = \sqrt{\alpha}\lambda^{2^{n-1}} = [\alpha] \lambda^{2^{n-1}}\\ \implies [a_{n-1},a_n ] &\le \sqrt{\alpha\lambda^{2^{n-1}}+\sqrt{\alpha}\lambda^{2^{n-1}}} =\sqrt{\alpha+\sqrt{\alpha}}\lambda^{2^{n-2}} = [\alpha,\alpha]\lambda^{2^{n-2}}\\ \implies [a_{n-2},a_{n-1},a_n]&\le \sqrt{\alpha\lambda^{2^{n-2}} + \sqrt{\alpha+\sqrt{\alpha}}\lambda^{2^{n-2}}} = [\alpha,\alpha,\alpha]\lambda^{2^{n-3}}\\ &\;\vdots\\ \implies [a_1,\ldots,a_n] & \le \underbrace{ [ \alpha,\ldots,\alpha ] }_{n\text{ terms}} \lambda\\ \implies [a_1, \ldots,a_n] & \le [ \alpha, \alpha, \ldots ]\lambda = \frac{1 + \sqrt{1+4\alpha}}{2}\lambda \end{align}$$

Since $n \le \sqrt{2}^{2^n-2}$, we can take $\alpha = \frac12$ and $\lambda = \sqrt{2}$ to get:

$$[1,2,\ldots,n] \le \underbrace{ [ \frac12,\ldots,\frac12 ]}_{n\text{ terms}} \lambda \le \frac{1+\sqrt{3}}{\sqrt{2}} \sim 1.931851 $$

To get a better bound, observe for any $m, k \in \mathbb{Z}_{+}$, we have:

$$ m + k - 1 \le \frac{m^2}{m+1}\left(\sqrt{\frac{m+1}{m}}\right)^{2^k}$$

Using the same approach as above, we get: $$[m,m+1,m+2,\ldots] \le \frac{\sqrt{m+1}+\sqrt{4m^2+m+1}}{2\sqrt{m}}$$

Take $m = 3$, we already get a bound accurate up to $O(10^{-2})$.

$$[3,4,\ldots] \le \frac{1+\sqrt{10}}{\sqrt{3}} \implies [1,2,3,\ldots] \le \sqrt{1+\sqrt{2+\frac{1+\sqrt{10}}{\sqrt{3}}}} \sim 1.760214368 $$

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Take a positive sequence $\{a_n\}$ and a constant $c>0$ such that $\sqrt{a_{n+1}}<ca_n$.

Set $b_n=\sqrt{a_1+\sqrt{a_2+\cdots \sqrt{a_n}}}$. By induction, $$\log_{c+1} \left(\frac{b_n}{\sqrt{a_1}}\right)<\sum_{i=1}^{n-1}2^{-i}<1$$ So $b_n<(c+1)\sqrt{a_1}$ and $b_n$ is monotonic increasing; by the Monotone Convergence Theorem, $\lim_{n\to\infty}b_n$ exists. Taking $a_n=n$ and $c>\sqrt{2}$ answers this question.

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By linear approximations.

Since the sequence is increasing, it converges iff it doesn't go to infinity. Therefore we need only to construct an upper bound.

Notice that all tangents to $\sqrt{x}$ are always above $\sqrt{x}$. The slope of each tangent is $\frac12\frac1{\sqrt x}$, which is at most $\frac 1 2$ when $x\geq 1$.

So $$\sqrt{1 + \sqrt{2 + \sqrt{3 + \dotsc}}} \leq 1 + \frac12(\sqrt 2 + \frac12(\sqrt{3} + \frac12(\dotsc))) = \sum_{i=1}^\infty \frac{\sqrt i}{2^{i-1}}$$ and it's easier to see that that converges.

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    $\begingroup$ The last term is just $2 \text{Li}_{-\frac{1}{2}}\left(\frac{1}{2}\right)\approx 2.69451$ $\endgroup$ – Claude Leibovici Oct 17 '17 at 6:44
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The proofs previous to this one have all been non-constructive. They all state an upper bound, observe that the sequence is monotonically increasing, and then appeal to the Monotone Convergence Theorem. This type of argument cannot provide a computable error bound. The following does.

Let $L_n = \sqrt{1 + \sqrt{2 + \dots\sqrt{n}}}$ and $U_n = \sqrt{1 + \sqrt{2 + \dots \sqrt{n}\phi}}$, where $\phi$ is the golden ratio. We assume $n > 1$.

Observe that since $\phi = \sqrt{1 + \sqrt{1 + \dots}}$, we have that $\sqrt{n}\phi = \sqrt{n^{2^0} + \sqrt{n^{2^1} + \sqrt{n^{2^2} + \dots}}}$. Comparing $n^{2^1}$ to $n+1$ and $n^{2^2}$ to $n+2$ and so on, we have that $L_i < U_j$ for all $i$ and $j$.

Observe that $L_n$ is monotonically increasing.

Define $f_n(x) = \sqrt{1 + \sqrt{2 + \dots\sqrt{n + x}}}$. It then follows that $L_{n} = f_n(0)$ and $U_{n+1} = f_n(\phi\sqrt{n+1})$.

So for $i \geq 0$, we have $$\begin{align} &L_n &\leq L_{n + i} &\leq U_{n+1} \\\iff &0 &\leq L_{n + i} - L_n &\leq U_{n+1} - L_n \\&&&= f_n(\phi\sqrt{n+1}) - f_n(0) \\&&&\leq f_n'(0) \times \phi\sqrt{n+1} \tag{by Mean Value Theorem} \end{align}$$

The last inequality follows from the Mean Value Theorem. Using routine calculus, we have that $$\begin{align} f_n'(0) &= \frac{1}{2^n\sqrt{n} \cdot\sqrt{(n-1) + \sqrt{n}} \dots \sqrt{1 + \sqrt{2 + \dots{\sqrt{(n-1) + \sqrt{n}}}}}} \\&\leq \frac{1}{2^n\sqrt{n!}}. \end{align}$$

Therefore $$\begin{align} &0 &\leq L_{n + i} - L_n &\leq \frac{\phi\sqrt{n+1}}{2^n\sqrt{n!}}. \end{align}$$

The rightmost side is exponentially decreasing. It follows then that the sequence $L_n$ is Cauchy, and must therefore converge.

PS. The following approximation $\ell_n = [1,2,\dotsc,n-1,n+1] = f_n(1)$ is a lower bound which is stronger than $L_n = [1, 2, \dotsc, n-1, n] = f_n(0)$. The error is then at most $\frac{1}{2^n\sqrt{n!}}$ (true for $n \geq 2$). In particular, $\ell_3 = \sqrt{3} \approx 1.7$ is accurate to 2 significant figures.

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  • $\begingroup$ I am verry sorry, I just clicked wrong. Btw I wouldnt consider myself a coward! I tried to undo my vote, but I cant until you edit your answer, sorry $\endgroup$ – TwoStones Jul 6 at 17:20
  • $\begingroup$ Sorry again! It was really just a mistake of mine $\endgroup$ – TwoStones Jul 6 at 17:22
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    $\begingroup$ Thanks, this is really nice. $\endgroup$ – MJD Jul 7 at 21:52

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