Would anyone be so kind to tell me if my answer to a problem is correct? Thank you! Happy new year!

The problem appeared on the UW-Madison Analysis Qual in January 2016.

Give an example of a non-empty closed subset of $L^2([0,1])$ that does not contain a vector of smallest norm.

I know an example that works. It's $\frac{n+1}{n}e_n$ where $e_n$ is an orthonormal basis for the $L^2([0,1])$ space. You can choose $e_n=\sqrt{2}\sin(n\pi x)$.

**What I would like to know is the following:**

My initial guess was $\{\sqrt{n+1}\chi_{[0,1/n]} : n\in \mathbb{N}\}$ where $\chi_{[0,1/n]}$ is the characteristic function of $[0,1/n]$. I thought this example worked because the $L^2$ norm of each term is $\sqrt{\frac{n+1}{n}}$, which is decreasing in $n$, so the smallest norm is never achieved. **Does this example work? Is the set closed?**

I have done some computation and realized that the distances between arbitrary terms does go to zero, which means we can't use the same argument as the orthonormal basis example.

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## Best Answer

Well, it is closed. Assume a sequence from $\{\sqrt{n+1}\chi_{[0,1/n]}\}$ were convergent in $L^2$ and denote the limit by $g$. The sequence must have a subsequence that converges to $g$ a.e. Suppose $g$ is not in the set $\{\sqrt{n+1}\chi_{[0,1/n]}\}$. Since $\sqrt{n+1}\chi_{[0,1/n]}$ converges to zero function a.e. as $n\to\infty$, any non-constant convergent sequence must converge to the zero function a.e. So $g = 0$ a.e. However, $||\sqrt{n+1}\chi_{[0,1/n]}||_2>1$ and $||g||_2=0$, so there can't be an $L^2$ convergence. Contradiction. $g$ is in the set and the set is closed.