\(\eta_0\), written as "eta-zero", is a small countable ordinal defined as the first fixed point of the function \(\alpha\mapsto\)\(\,\zeta\)\(_\alpha\). It's equal to \(\varphi(3,0)\) in Veblen's function, \(\psi_0(\Omega^3)=\psi_0(\psi_1(\psi_1(0)+\psi_1(0)))\) in Buchholz's function, and \(\psi(\Omega^2)\) in Madore's function.
- Bird, Chris. Beyond Nested Arrays I (p.2). Retrieved 2021-06-17.