Definition 4.7.3.10 (Regular Cardinals). Let $\kappa $ be a cardinal. We say that $\kappa $ is regular if it is infinite and $\mathrm{cf}(\kappa ) = \kappa $. Here $\mathrm{cf}(\kappa )$ denotes the cofinality of $\kappa $ (Definition 4.7.1.28). We say that $\kappa $ is singular if it is infinite but not regular.
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