Bourbaki

Proposition 7

Tag 08E4, permanent. Cite it as [Bourbaki, Tag 08E4].

Functions of a Real Variable, chapter V, § 1. COMPARISON OF FUNCTIONS ON A FILTERED SET

If ff and gg are two functions in H(F,V)\mathscr{H}(\mathfrak{F}, V) then the relation f−g≪ff - g \ll f is equivalent to f−g≪gf - g \ll g.

Indeed, f−g≪ff - g \ll f implies that for every ε>0\varepsilon > 0 there exists an X∈FX \in \mathfrak{F} such that ∥f(x)−g(x)∥≤ε∥f(x)∥\|f(x) - g(x)\| \leq \varepsilon \|f(x)\| for every x∈Xx \in X. But then (1−ε)∥f(x)∥≤∥g(x)∥(1 - \varepsilon) \|f(x)\| \leq \|g(x)\|, and consequently f≼gf \preccurlyeq g, whence (V, p. 215, prop. 4) f−g≪gf - g \ll g.

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

Cited by

Nothing.

Cites

Transcribed from the printed volume. Not translated.