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Math and science::Topology

Metric space. Sequence convergence

Sequence convergence, definition

Let X be a metric space, let x0X and let (an)n=1 be a sequence in X.

(an)n=1 is said to converge to x0 iff

[ lim??=? ]

[unfolding the definition of limits, this becomes...]



This next result should be internalized

Lemma. Closed iff [some property of sequences...]

Let X be a metric space and VX.

Then V is closed in X [some property of sequences...]

The proof was not immediately obvious to me. I did, however, find a way to visualize it, which I think makes the line of reasoning clear. Part of why the proof is important, I think, is that there are only two notions available from which the result needs to be derived: the definition of open sets and the definition of sequence convergence. So, it should be simple, yet it escapes an immediately obvious proof. Thus, I feel there is a mode of thinking that has its essence somewhat distilled into the line of reasoning.