Sunday 30 December 2012

real analysis - Prob. 3, Chap. 3 in Baby Rudin: If $s_1 = sqrt{2}$, and $s_{n+1} = sqrt{2 + sqrt{s_n}}$, what is the limit of this sequence?

Here's Prob. 3, Chap. 3 in the book Principles of Mathematical Analysis by Walter Rudin, 3rd edition:




If $s_1 = \sqrt{2}$, and $$s_{n+1} = \sqrt{2 + \sqrt{s_n}} \ \ (n = 1, 2, 3, \ldots),$$ prove that $\left\{ s_n \right\}$ converges, and that $s_n < 2$ for $n = 1, 2, 3, \ldots$.





My effort:



We can show that $\sqrt{2} \leq s_n \leq 2$ for all $n = 1, 2, 3, \ldots$. [Am I right?]



Then we can also show that $s_n < s_{n+1}$ for all $n = 1, 2, 3, \ldots$. [Am I right?]



But how to calculate the exact value of the limit? Where does this sequence occur in applications?

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