If every convergent subsequence converges to the same limit, does the whole sequence converge to that limit

convergence-divergencemetric-spacessequences-and-series

$(E,d)$ be a metric space and $(x_n)_{n\in\mathbb N}\subseteq E$ be relatively compact.

Assume that there is a $x\in E$ such that every convergent subsequence converges to $x$. Does it already follow that $(x_n)_{n\in\mathbb N}$ is convergent and converges to $x$?

The convergence of $(x_n)_{n\in\mathbb N}$ to $x$ is equivalent to the assertion that every aribtrary subsequence is convergent and converges to $x$; but we only know that every subsequence, for which we a priori assume that it is convergent, converges to $x$.

Best Answer

Let the original sequence be $\sigma$, and let $K=\operatorname{cl}\{x_n:n\in\Bbb N\}$; by hypothesis $K$ is compact. If $\sigma$ does not converge to $x$, there are an $\epsilon>0$ and a subsequence $\sigma_1=\langle x_{n_k}:k\in\Bbb N\rangle$ such that $d(x_{n_k},x)\ge\epsilon$ for all $k\in\Bbb N$. Clearly $\sigma_1$ is a sequence in the compact set $K$, so it has a convergent subsequence $\sigma_2=\langle x_{n_{k_i}}:i\in\Bbb N\rangle$. But $\sigma_2$ is also a subsequence of $\sigma$, so by hypothesis it converges to $x$, contradicting the fact that $d(x_{n_{k_i}},x)\ge \epsilon$ for each $i\in\Bbb N$. Thus, $\sigma$ must converge to $x$.

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