By the AC (in the form of the Well-Ordering Principle), every set \(A\) can be well-ordered, hence it is bijectable with a unique cardinal, called the cardinality of \(A\). Given two cardinals \(\kappa\) and \(\lambda\), the sum \(\kappa +\lambda\) is defined as the cardinality of the set consisting of the union of any two disjoint sets, one of cardinality \(\kappa\) and one of cardinality \(\lambda\). And the product \(\kappa \cdot \lambda\) is defined as the cardinality of the Cartesian product \(\kappa \times \lambda\). The operations of sum and product of infinite cardinals are trivial, for if \(\kappa\) and \(\lambda\) are infinite cardinals, then \(\kappa +\lambda =\kappa \cdot \lambda = maximum \{ \kappa ,\lambda\}\).