Cantor's ordinal

Cantor's ordinal \(\zeta_0\) (pronounced "zeta-zero", "zeta-null" or "zeta-nought") is a small countable ordinal, defined as the first fixed point of the function \(\alpha \mapsto \varepsilon_\alpha\).

It is equal to \(\varphi(2,0)\) using the Veblen function and to \(\psi(\Omega)\), using Madore's \(\psi\) function.