Saturday, August 4, 2018

real analysis - Limit of $left(1-frac{1}{2^2}right)left(1-frac{1}{3^3}right)dots left(1-frac{1}{n^n}right)$ as $nto infty$



So I'm trying to solve the following limit:



$$\lim_{n \to \infty}\left(1-\frac{1}{2^2}\right)\left(1-\frac{1}{3^3}\right)\dots \left(1-\frac{1}{n^n}\right)$$



Now, I tried getting the squeeze theorem around this one, since it does feel like something for the squeeze theorem. The upper bound is obviously $1$, but since each term decreases the product, it may seem like this approaches zero?


Answer



$$

\begin{align}
\prod_{n=2}^\infty\left(1-\frac1{n^n}\right)
&\ge\prod_{n=2}^\infty\left(1-\frac1{n^2}\right)\\
&=\lim_{m\to\infty}\prod_{n=2}^m\frac{n-1}n\frac{n+1}n\\
&=\lim_{m\to\infty}\prod_{n=2}^m\frac{n-1}n\prod_{n=2}^m\frac{n+1}n\\
&=\lim_{m\to\infty}\frac1m\frac{m+1}2\\[6pt]
&=\frac12
\end{align}
$$
So the product converges (that is, it is bounded away from $0$). However, numerically, the product is approximately $0.71915450096501024665446931$ and the Inverse Symbolic Calculator does not find anything for this number.



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