Quine atom
= Quine atom
{c}
{title2=$q=\{q\}$}
A Quine atom is a set whose sole member is itself. It violates the <Axiom of foundation> while respecting the <axiom of extensionality>. Swapping pairwise distinct old objects $a_n$ with their singletons in a <Rieger-Bernays permutation model> supplies many Quine atoms, provided those transpositions have disjoint supports. For example, the old ordinals $a_n=\omega+n$ and their singletons are disjoint families.