Example 4.7.3.14. Let $\aleph _1$ denote the first uncountable cardinal (Example 4.7.2.12). Then $\aleph _1$ is regular: that is, the collection of countable sets is closed under the formation of countable disjoint unions. This is a special case of Example 4.7.3.13, since $\aleph _1 = \aleph _0^{+}$.
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$