Axiom1 of Extension (аксиома на удължаването / разширението):  a set is determined by what its elements are - not in the order in which they might be listed or the fact that some elements might be listed more than once.

We say that sets and are equal if and only if и .

Set equality is also a Relation between two sets.


  1. axiom: a statement that is taken to be true to serve as a starting point for further reasoning and arguments