Math 2.0

Sequences, series, summability Definition

Limit of a sequence

Builds on Real sequence

Definition

Let (an)(a_n) be a real sequence and let LL be a real number. The sequence (an)(a_n) converges to LL if the following holds for each open interval UU that contains LL. There is a natural number NN such that an∈Ua_n \in U for all n≥Nn \ge N. Then LL is a limit of (an)(a_n), with the notation lim⁡n→∞an=L\lim_{n \to \infty} a_n = L or an→La_n \to L as n→∞n \to \infty.

Remarks

The epsilon-N definition of the limit of a sequence is an equivalent form, with intervals (L−ε,L+ε)(L - \varepsilon, L + \varepsilon). A sequence has at most one limit, by the uniqueness of limits. So the notation lim⁡n→∞an\lim_{n \to \infty} a_n is well defined. The same definition works in each topological space, with open neighbourhoods of LL in place of open intervals. In Mathlib, the limit is the statement Filter.Tendsto a Filter.atTop (nhds L).

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  1. Sequence definition
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Further reading

  1. Web https://en.wikipedia.org/wiki/Limit_of_a_sequence, Real numbers
  2. Web https://en.wikipedia.org/wiki/Limit_of_a_sequence, Topological spaces
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