Example 1

Tag 01LB, permanent. Cite it as [Bourbaki, Tag 01LB].

Topologie algébrique, chapter I, § 1. Produits fibrés et carrés cartésiens

Let X and $X'$ be B-spaces; then the mapping $(x, x')\mapsto (x', x)$ defines a B-isomorphism of $X\times_BX'$ onto $X'\times_BX$ .

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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