Bourbaki

Remark

Tag 0BR6, permanent. Cite it as [Bourbaki, Tag 0BR6].

General Topology, chapter IV, § 5. Real-valued functions

If in particular we have f(x)<g(x)f(x) < g(x) for all x∈Xx \in X (or only for all points of a set of the filter R‾\overline{\mathbf{R}}) we can infer, by Theorem 1, that lim⁡Ff≤lim⁡Fg\lim_{\mathfrak{F}} f \leq \lim_{\mathfrak{F}} g; but we cannot infer the strong inequality lim⁡Ff<lim⁡Fg\lim_{\mathfrak{F}} f < \lim_{\mathfrak{F}} g. For example, if we take XX to be the set N\mathbf{N} of natural numbers, filtered by the Fréchet filter, and if f(n)=0f(n) = 0 and g(n)=1/ng(n) = 1/n, then f(n)<g(n)f(n) < g(n) for all nn, but lim⁡n→∞f(n)=lim⁡n→∞f(n)=0\lim_{n \to \infty} f(n) = \lim_{n \to \infty} f(n) = 0. Thus we lose strictness when we pass to the limit in a strict inequality.

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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Cites

Transcribed from the printed volume. Not translated.