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 \(\omega \uparrow\uparrow\uparrow 2\)
 * \(2^{\omega^\omega}\).

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.

The limit of the epsilon numbers is the first fixed point of \(\alpha \mapsto \varepsilon_\alpha\). This ordinal is called \(\zeta_0\) (zeta-zero), 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:


 * \(\phi(0,\alpha) = \omega^\alpha\)
 * \(\phi(\beta,\alpha)\) is the \(\alpha\)th fixed point of \(\phi_\gamma\) for all \(\gamma<\beta\)

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