Ε₀

\(\varepsilon_0\) (pronounced "epsilon-zero", "epsilon-null" or "epsilon-nought") is a small countable ordinal, defined as the first fixed point of the function \(\alpha \mapsto \omega^\alpha\). It can also be equivalently defined in several other ways:


 * Smallest ordinal not expressible in using strictly smaller exponents.
 * The proof-theoretic ordinal of and ACA0 (arithmetical comprehension, a subsystem of second-order arithmetic).
 * Informal visualizations: \(\omega^{\omega^{\omega^⋰}}\) or \(\omega \uparrow\uparrow \omega\) or \(\omega \uparrow\uparrow\uparrow 2\)
 * The second fixed point of \(x\mapsto2^x\).
 * \(\varphi(1,0)\) using the Veblen function
 * \(\psi_0(\Omega)\) using Buchholz notation
 * \(\psi(0)\) using Madore’s notation

Using the Wainer hierarchy:


 * \(f_{\varepsilon_0}(n) \approx X \uparrow\uparrow X\ \&\ n\) (fast-growing hierarchy)
 * \(H_{\varepsilon_0}(n) \approx X \uparrow\uparrow X\ \&\ n\) (Hardy hierarchy)
 * \(g_{\varepsilon_0}(n) = n \uparrow\uparrow n = n \uparrow\uparrow\uparrow 2\) (slow-growing hierarchy)

\(f_{\varepsilon_0}(n)\) is comparable to the Goodstein function and Goucher's T(n) function.

Higher epsilon numbers and the Veblen hierarchy
The function \(\alpha \mapsto \varepsilon_\alpha\) enumerates the fixed points of the exponential map \(\alpha \mapsto \omega^\alpha\). Thus \(\varepsilon_1\) is the next fixed point of the exponential map. Formally:


 * \(\varepsilon_0=\text{min}\{\alpha|\alpha=\omega^\alpha\}=\text{sup}\{0,1,\omega, \omega^\omega, \omega^{\omega^\omega},...\}\)
 * \(\varepsilon_{\alpha+1}=\text{min}\{\beta|\beta=\omega^\beta\wedge\beta>\varepsilon_\alpha\}=\text{sup}\{\varepsilon_\alpha+1,\omega^{\varepsilon_\alpha+1}, \omega^{\omega^{\varepsilon_\alpha+1}},...\}\)
 * \(\varepsilon_{\alpha}=\text{sup}\{\varepsilon_{\beta}|\beta<\alpha\}\) if \(\alpha\) is a limit ordinal.

This definition gives the following fundamental sequences for epsilon numbers:
 * if \(\alpha=\varepsilon_0\) then \(\alpha[0]=0\) and \(\alpha[n+1]=\omega^{\alpha[n]}\)
 * if \(\alpha=\varepsilon_{\beta+1}\) then \(\alpha[0]=\varepsilon_\beta+1\) and \(\alpha[n+1]=\omega^{\alpha[n]}\)
 * if \(\alpha=\varepsilon_{\beta}\) and \(\beta\) is a limit ordinal then \(\alpha[n]=\varepsilon_{\beta[n]}\)

The first fixed point of \(\alpha \mapsto \varepsilon_\alpha\) is called \(\zeta_0\) (zeta-zero) or Cantor's ordinal, and \(\zeta_\alpha\) enumerates the fixed points of \(\alpha \mapsto \varepsilon_\alpha\).

Since we do not have an infinite number of Greek letters, we generalize this using a series of functions that form the Veblen hierarchy. Each function enumerates the fixed points of the previous one. Formally:


 * \(\varphi_0(\alpha) = \omega^\alpha\)
 * \(\varphi_\beta(\alpha)\) is the \((1+\alpha)\)th fixed point of \(\varphi_\gamma\) for all \(\gamma < \beta\)

The first ordinal inaccessible through this two-argument Veblen hierarchy is the Feferman–Schütte ordinal.