It is an interesting task to try finding the limit of nested square root expressions.

$$\lim_{n \to \infty}\left( 1 + \sqrt{2 + \sqrt{3+ ... + \sqrt {n + \sqrt{n+1}}}}\right)$$

How to solve this one?

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  • $\begingroup$ Relevant: This and this. $\endgroup$ – Ben Jan 28 '14 at 12:19

This is the square of the Nested Radical Constant, which converges, but is not known to possess a closed form. See also Somos's Quadratic Recurrence Constant.

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Convergence of this nested radical expression can be seen by Herschfeld's convergence test (see Herschfeld, On Infinite Radicals. Amer. Math. Monthly 42, 419-429, 1935.):

Theorem: For $0<p<1$ and $a_n\ge 0$, the limit $$\lim_{n\rightarrow\infty} a_1+(a_2+(\cdots+(a_n)^p)^p)^p$$ exists if and only if the sequence $(a_n^{p^n})_n$ is bounded.

That reduces checking convergence to seeing that $a_n^{p^n}=n^{2^{-n}}$ is bounded, which is clear since

$$n^{2^{-n}}=e^{2^{-n}\log n}\longrightarrow 1$$

as $n\rightarrow\infty$.

However, no closed form is known to express the limit.

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  • $\begingroup$ This is probably a long shot, but since you mentioned Herschfeld, I have to ask: is there any deeper meaning to his observation that continued fractions are nested radicals of order $-1$ ? Some implications, perhaps ? Thank you. $\endgroup$ – Lucian Jan 28 '14 at 13:01
  • $\begingroup$ @Lucian No, as far as I remember he just remarks that and nobody really knows what that is good for. At least I'm not aware of any implications. $\endgroup$ – J.R. Jan 28 '14 at 13:39

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