Bourbaki

Corollary

Tag 03JM, permanent. Cite it as [Bourbaki, Tag 03JM].

Theory of Sets, chapter III, § 1. Order relations. Ordered sets

Let (xι)ι∈I(x_\iota)_{\iota \in I} be a family of elements of an ordered set E\mathrm{E} which has a least upper bound in E\mathrm{E}. If J\mathrm{J} is a subset of I\mathrm{I} such that the family (xι)ι∈J(x_\iota)_{\iota \in J} has a least upper bound in E\mathrm{E}, we have sup⁡ι∈Jxι⩽sup⁡ι∈Ixι\sup\limits_{\iota \in J} x_\iota \leqslant \sup\limits_{\iota \in I} x_\iota.

Bourbaki sets the proof directly after the statement, and the corpus does not separate them, so what is above is the statement and its proof together.

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Transcribed from the printed edition, 2004, Springer. Not translated.