Proposition 4

Let $(E_\iota)_{\iota \in I}$ be a family of sets, P their product, and S their sum, and let $\mathfrak{a}_\iota$ be the cardinal of $E_\iota$. Then the cardinal of P (resp. S) is the cardinal product (resp. cardinal sum) of the family $(\mathfrak{a}_\iota)_{\iota \in I}$.

For there exists a bijection of P (resp. S) onto the product (resp. sum) of the sets $(\mathfrak{a}_\iota)$ (Chapter II, § 4, no. 8, Proposition 10, and § 5, no. 7, Corollary to Proposition 11).