Hereditarily countable set
= Hereditarily countable set
{wiki=Hereditarily_countable_set}
A set is called **hereditarily countable** if it is countable, and all of its elements (and their elements, recursively) are also countable. In more formal terms, a set \\( A \\) is hereditarily countable if: 1. \\( A \\) is countable. 2. Every element of \\( A \\) is countable. 3. Every element of every element of \\( A \\) is countable, and so on.