Bourbaki

Exercise 5, § 3

Tag 03L7, permanent. Cite it as [Bourbaki, Tag 03L7].

Theory of Sets, III, 230

Let (λι)ι∈I(\lambda_\iota)_{\iota \in I} be a family of order-types (§2§ 2, Exercise 13), indexed by an ordered set I. Show that

Card(∑ι∈Iλι)=∑ι∈ICard (λι)\mathrm{Card} \left( \sum_{\iota \in I} \lambda_\iota \right) = \sum_{\iota \in I} \mathrm{Card}\ (\lambda_\iota)

and that, if I is well-ordered,

Card(Pι∈Iλι)=Pι∈ICard (λι).\mathrm{Card} \left( \mathop{\mathbf{P}}_{\iota \in I} \lambda_\iota \right) = \mathop{\mathbf{P}}_{\iota \in I} \mathrm{Card}\ (\lambda_\iota).
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Transcribed from the printed volume. Not translated.