Buchholz's function

Buchholz's psi functions are ordinal collapsing functions created by German mathematician Wilfried Buchholz. Although there are many Buchholz's psi functions, we explain the most famous one in this community, which is a hierarchy of single-argument functions \(\psi_\nu(\alpha)\) introduced in 1986, because the other ones are not currently used in this community. These functions are a simplified version of Feferman's \(\theta\) functions, but nevertheless have the same strength as those.

Definition
Buchholz defined his functions as follows:


 * \(C_\nu^0(\alpha) = \Omega_\nu\),
 * \(C_\nu^{n+1}(\alpha) = C_\nu^n(\alpha) \cup \{\gamma | P(\gamma) \subseteq C_\nu^n(\alpha)\} \cup \{\psi_\mu(\xi) | \xi \in \alpha \cap C_\nu^n(\alpha) \wedge \xi \in C_\mu(\xi) \wedge \mu \leq \omega\}\),
 * \(C_\nu(\alpha) = \bigcup_{n < \omega}C_\nu^n (\alpha)\),
 * \(\psi_\nu(\alpha) = \min\{\gamma | \gamma \not\in C_\nu(\alpha)\}\),

where \(\omega\) is the smallest infinite ordinal,

\(\Omega_\nu=\left\{\begin{array}{lcr}1\text{ if }\nu=0\\\aleph_\nu\text{ if }\nu>0\\\end{array}\right.\)

and \(P(\gamma)=\{\gamma_1,\cdots,\gamma_k\}\) is the set of additive principal numbers in form \(\omega^\xi\),

\(P=\{\alpha\in \textrm{On}: 0<\alpha \wedge \forall \xi, \eta < \alpha (\xi+\eta < \alpha)\}=\{\omega^\xi: \xi \in \textrm{On}\}\),

the sum of which gives this ordinal \(\gamma\):

\(\gamma=\alpha_1+\alpha_2+\cdots+\alpha_k\) where \(\alpha_1\geq\alpha_2\geq\cdots\geq\alpha_k\) and \(\alpha_1,\alpha_2,\cdots,\alpha_k \in P(\gamma)\).

Note: Greek letters always denotes ordinals. \(\textrm{On}\) denotes the class of all ordinals. The convention \(\Omega_0\) depends on the context even if we are focussing on Buchholz's function. Actually, Buchholz himself uses \(\Omega_0\) as both of shorthands of \(1\) and \(\omega\).

The limit of this notation is Takeuti-Feferman-Buchholz ordinal.

Properties
Buchholz showed following properties of those functions:


 * \(\psi_\nu(0)=\Omega_\nu\),
 * \(\psi_\nu(\alpha)\in P\),
 * \(\psi_\nu(\alpha+1)=\text{min}\{\gamma\in P: \psi_\nu(\alpha)<\gamma\}\text{ if }\alpha\in C_\nu(\alpha)\),
 * \(\Omega_\nu\le\psi_\nu(\alpha)<\Omega_{\nu+1}\),
 * \(\alpha\le\beta\Rightarrow\psi_\nu(\alpha)\le\psi_\nu(\beta)\),
 * \(\psi_0(\alpha)=\omega^\alpha \text{ if }\alpha<\varepsilon_0\),
 * \(\psi_\nu(\alpha)=\omega^{\Omega_\nu+\alpha} \text{ if }\alpha<\varepsilon_{\Omega_\nu+1} \text{ and } \nu\neq 0\),
 * \(\theta(\varepsilon_{\Omega_\nu+1},0)=\psi_0(\varepsilon_{\Omega_\nu+1})\) for \(0<\nu\le\omega\).

Explanation
Buchholz is working in Zermelo–Fraenkel set theory, that means every ordinal \(\alpha\) is equal to set \(\{\beta|\beta<\alpha\}\). Then condition \(C_\nu^0(\alpha)=\Omega_\nu\) means that set \(C_\nu^0(\alpha)\) includes all ordinals less than \(\Omega_\nu\) in other words \(C_\nu^0(\alpha)=\{\beta|\beta<\Omega_\nu\}\).

The condition \(C_\nu^{n+1}(\alpha) = C_\nu^n(\alpha) \cup \{\gamma | P(\gamma) \subseteq C_\nu^n(\alpha)\} \cup \{\psi_\mu(\xi) | \xi \in \alpha \cap C_\nu^n(\alpha) \wedge \mu \leq \omega\}\) means that set \(C_\nu^{n+1}(\alpha)\) includes:


 * 1) all ordinals from previous set \(C_\nu^n(\alpha)\),
 * 2) all ordinals that can be obtained by summation the additively principal ordinals from previous set \(C_\nu^n(\alpha)\),
 * 3) all ordinals that can be obtained by applying ordinals less than \(\alpha\) from the previous set \(C_\nu^n(\alpha)\) as arguments of functions \(\psi_\mu\), where \(\mu\le\omega\).

That is why we can rewrite this condition as:

\(C_\nu^{n+1}(\alpha) = \{\beta+\gamma,\psi_\mu(\eta)|\beta, \gamma,\eta\in C_{\nu}^n(\alpha)\wedge\eta<\alpha \wedge \mu \leq \omega\}\).

Thus union of all sets \(C_\nu^n (\alpha)\) with \(n<\omega\) i.e. \(C_\nu(\alpha) = \bigcup_{n < \omega} C_\nu^n (\alpha)\) denotes the set of all ordinals which can be generated from ordinals \(<\aleph_\nu\) by the functions + (addition) and \(\psi_{\mu}(\xi)\), where \(\mu\le\omega\) and \(\xi<\alpha\).

Then \(\psi_\nu(\alpha) = \min\{\gamma | \gamma \not\in C_\nu(\alpha)\}\) is the smallest ordinal that does not belong to this set.

Examples
Consider the following examples:

\(C_0^0(\alpha)=\{0\} =\{\beta:\beta<1\}\),

\(C_0(0)=\{0\}\) (since no functions \(\psi(\eta<0)\) and 0+0=0).

Then \(\psi_0(0)=1\).

\(C_0(1)\) includes \(\psi_0(0)=1\) and all possible sums of natural numbers:

\(C_0(1)=\{0,1,2,...,\text{googol}, ...,\text{TREE(googol)},...\}\).

Then \(\psi_0(1)=\omega\) - first transfinite ordinal, which is greater than all natural numbers by its definition.

\(C_0(2)\) includes \(\psi_0(0)=1, \psi_0(1)=\omega\) and all possible sums of them.

Then \(\psi_0(2)=\omega^2\).

For \(C_0(\omega)\) we have set \(C_0(\omega)=\{0,\psi(0)=1,...,\psi(1)=\omega,...,\psi(2)=\omega^2,...,\psi(3)=\omega^3,...\}\).

Then \(\psi_0(\omega)=\omega^\omega\).

For \(C_0(\Omega)\) we have set \(C_0(\Omega)=\{0,\psi(0)=1,...,\psi(1)=\omega,...,\psi(\omega)=\omega^\omega,...,\psi(\omega^\omega)=\omega^{\omega^\omega},...\}\).

Then \begin{eqnarray*} & & \psi_0(\varepsilon_0)=\psi_0(\varepsilon_0+1) \\ & = & \cdots=\psi_0(\textrm{insert any countable ordinal above } \varepsilon_0 \textrm{which you like very much}) \\ & = & \cdots = \psi_0(\Omega)=\varepsilon_0. \end{eqnarray*} For \(C_0(\Omega+1)\) we have set \(C_0(\Omega)=\{0,1,...,\psi_0(\Omega)=\varepsilon_0,...,\varepsilon_0+\varepsilon_0,...\psi_1(0)=\Omega,...\}\).

Then \(\psi_0(\Omega+1)=\varepsilon_0\omega=\omega^{\varepsilon_0+1}\).

\(\psi_0(\Omega2)=\varepsilon_1\),

\(\psi_0(\Omega^2)=\zeta_0\),

\(\psi_0(\Omega^\alpha(1+\beta)) = \varphi(\alpha,\beta)\),

\(\psi_0(\Omega^\Omega)=\Gamma_0=\theta(\Omega,0)\), using Feferman theta-function,

\(\psi_0(\Omega^{\Omega^\Omega})\) is large Veblen ordinal,

\(\psi_0(\varepsilon_{\Omega+1})=\theta(\varepsilon_{\Omega+1},0)\).

Now let's research how \(\psi_1\) works:

\(C_1^0(\alpha)=\{\beta:\beta<\Omega_1\}=\{0,\psi(0)=1,2,...\text{googol},...,\psi_0(1)=\omega,...,\psi_0(\Omega)=\varepsilon_0,...\)

\(...,\psi_0(\Omega^\Omega)=\Gamma_0,...,\psi(\Omega^{\Omega^\Omega+\Omega^2}),...\}\) i.e. includes all countable ordinals.

\(C_1(\alpha)\) includes all possible sums of all countable ordinals. Then

\(\psi_1(0)=\Omega_1\) first uncountable ordinal which is greater than all countable ordinal by its definition i.e. smallest number with cardinality \(\aleph_1\).

\(C_1(1)=\{0,...,\psi_0(0)=\omega,...,\psi_1(0)=\Omega,...,\Omega+\omega,...,\Omega+\Omega,...\}\)

Then \(\psi_1(1)=\Omega\omega=\omega^{\Omega+1}\).

Then \(\psi_1(2)=\Omega\omega^2=\omega^{\Omega+2}\),

\(\psi_1(\psi_0(\Omega))=\Omega\varepsilon_0=\omega^{\Omega+\varepsilon_0}\),

\(\psi_1(\psi_0(\Omega^\Omega))=\Omega\Gamma_0=\omega^{\Omega+\Gamma_0}\),

\(\psi_1(\psi_1(0))=\psi_1(\Omega)=\Omega^2=\omega^{\Omega+\Omega}\),

\(\psi_1(\psi_1(\psi_1(0)))=\omega^{\Omega+\omega^{\Omega+\Omega}}=\omega^{\Omega\cdot\Omega}=(\omega^{\Omega})^\Omega=\Omega^\Omega\),

\(\psi_1^4(0)=\Omega^{\Omega^\Omega}\),

\(\psi_1(\Omega_2)=\varepsilon_{\Omega+1}\).

For case \(\psi(\Omega_2)\) the set \(C_0(\Omega_2)\) includes functions \(\psi_0\) with all arguments less than \(\Omega_2\) i.e. such arguments as \(0, \psi_1(0), \psi_1(\psi_1(0)), \psi_1^3(0),...\)

and then \(\psi_0(\Omega_2)=\psi_0(\psi_1(\Omega_2))=\psi_0(\varepsilon_{\Omega+1})\).

In general case: \(\psi_0(\Omega_{\nu+1})=\psi_0(\psi_\nu(\Omega_{\nu+1}))=\psi_0(\varepsilon_{\Omega_\nu+1})=\theta(\varepsilon_{\Omega_\nu+1},0)\).

We also can write:

\(\theta(\Omega_\nu,0)=\psi_0(\Omega_\nu^{\Omega_\nu})\) ( for \( 1\le\nu<\omega\)).

Ordinal notation
Buchholz defined an ordinal notation \((OT,<)\) associated to \(\psi\) as an array notation. We explain the original definition of \((OT,<)\).

We simultaneously define the sets \(T\) and \(PT\) of formal strings consisting of \(0\), \(D_v\) indexed by an \(v \in \omega+1\), braces, and commas in the following recursive way: Namely, \(T\) and \(PT\) are the smallest sets satisfying the conditions above. An element of \(T\) is called a term, and an element of \(PT\) is called a principal term. By the definition, \(T\) is a recursive set, \(PT\) is a recursive subset of \(T\), and every term is \(0\), a principal term, or an array of principal terms of length \(\geq 2\) in braces.
 * 1) \(0 \in T\) and \(0 \in PT\).
 * 2) For any \((v,a) \in (\omega+1) \times T\), \(D_va \in T\) and \(D_va \in PT\).
 * 3) For any \((a_i)_{i=0}^{k} \in PT^{k+1}\) with \(k \in \mathbb{N} \setminus \{0\}\), \((a_0,\ldots,a_k) \in T\). More precisely, this condition can be formulated without using ellipses in the following way:
 * 4) For any \((a_0,a_1) \in PT^2\), \((a_0,a_1) \in T\).
 * 5) For any \((a_0,a_1) \in T \times PT\), if there exists formal strings \(s\) such that \(a_0 = (s)\), then \((s,a_1) \in T\).

We also denote an \(a \in PT\) by \((a)\). Since the clause 3 in the definition of \(T\) and \(PT\) is applicable only to arrays of length \(\geq 2\), the additional convention does not cause serious ambiguity. By the convention, every term can be uniquely expressed as either \(0\) or a non-empty array of principal terms in braces.

We define a binary relation \(a < b\) on \(T\) in the following recursive way: By the definition, \(<\) is a recursive strict total ordering on \(T\). We abbreviate \((a < b) \lor (a = b)\) to \(a \leq b\). Although \(\leq\) itself is not a well-ordering, its restriction to a recursive subset \(OT \subset T\), which will be introduced later, forms a well-ordering.
 * 1) If \(b = 0\), then \(a < b\) is false.
 * 2) If \(a = 0\), then \(a < b\) is equivalent to \(b \neq 0\).
 * 3) Suppose \(a \neq 0\) and \(b \neq 0\).
 * 4) If \(a = D_ua'\) and \(b = D_vb'\) for some \(((u,a'),(v,b')) \in ((\omega+1) \times T)^2\), \(a < b\) is equivalent to that either one of the following holds:
 * 5) \(u < v\).
 * 6) \(u = v\) and \(a' < b'\).
 * 7) If \(a = (a_0,\ldots,a_n)\) and \(b = (b_0,\ldots,b_m)\) for some \((n,m) \in \mathbb{N}^2\) with \(1 \leq n+m\), \(a < b\) is equivalent to that either one of the following holds:
 * 8) \(n < m\) and \(a_i = b_i \) for any \(i \in \mathbb{N}\) with \(i \leq n\)
 * 9) There exists some \(k \in \mathbb{N}\) such that \(k \leq \min\{n,m\}\), \(a_k < b_k\), and \(a_i = b_i\) for any \(i \in \mathbb{N}\) with \(i < k\).

In order to define \(OT\), we define a subset \(G_ua \subset T\) for each \((u,a) \in (\omega+1) \times T\) in the following recursive way: By the definition, \(b \in G_ua\) is a recursive relation on \((u,a,b) \in (\omega+1) \times T^2\).
 * 1) If \(a = 0\), then \(G_ua = \emptyset\).
 * 2) Suppose that \(a = D_va'\) for some \((v,a') \in (\omega+1) \times T\).
 * 3) If \(u \leq v\), then \(G_ua = \{a'\} \cup G_ua'\).
 * 4) If \(u > v\), then \(G_ua = \emptyset\).
 * 5) If \(a = (a_0,\ldots,a_k)\) for some \((a_i)_{i=0}^{k} \in PT^{k+1}\) with \(k \in \mathbb{N} \setminus \{0\}\),\(G_ua = \bigcup_{i=0}^{k} G_ua_i\).

Finally, we define a subset \(OT \subset T\) in the following recursive way: By the definition, \(OT\) is a recursive subset of \(T\). An element of \(OT\) is called an ordinal term.
 * 1) \(0 \in OT\).
 * 2) For any \((v,a) \in (\omega+1) \times T\), \(D_va \in OT\) is equivalent to \(a \in OT\) and \(a' < a\) for any \(a' \in G_va\).
 * 3) For any \((a_i)_{i=0}^{k} \in PT^{k+1}\), \((a_0,\ldots,a_k) \in OT\) is equivalent to \((a_i)_{i=0}^{k} \in OT^{k+1}\) and \(a_k \leq \cdots \leq a_0\).

We define a map \begin{eqnarray*} o \colon OT & \to & C_0(\epsilon_{\Omega_{\omega}+1}) \\ a & \mapsto & o(a) \end{eqnarray*} in the following inductive way: Buchholz verified that the map \(o\) satisfies the following:
 * 1) If \(a = 0\), then \(o(a) = 0\).
 * 2) If \(a = D_va'\) for some \((v,a') \in (\omega+1) \times OT\), then \(o(a) = \psi_v(o(a'))\).
 * 3) If \(a = (a_0,\ldots,a_k)\) for some \((a_i)_{i=0}^{k} \in (OT \cap PT)^{k+1}\) with \(k \in \mathbb{N} \setminus \{0\}\), then \(o(a) = o(a_0) \# \cdots \# o(a_k)\), where \(\#\) denotes the descending sum of ordinals, which coincides with the usual addition by the definition of \(OT\).
 * The map \(o\) is an order-preserving bijective map with respect to \(<\) and \(\in\). In particular, \(<\) is a recursive strict well-ordering on \(OT\).
 * For any \(a \in OT\) satisfying \(a < D_10\), \(o(a)\) coincides with the ordinal type of \(<\) restricted to the countable subset \(\{x \in OT \mid x < a\}\).
 * The ordinal \(\psi_0(\varepsilon_{\Omega_{\omega}+1})\) coincides with the ordinal type of \(<\) restricted to the recursive subset \(\{x \in OT \mid x < D_10\}\). In particular, \((\{x \in OT \mid x < D_10\},<)\) is an ordinal notation equivalent to \(\psi_0(\varepsilon_{\Omega_{\omega}+1})\), and hence \((OT,<)\) is an ordinal notation whose ordinal type is strictly greater than the countable limit of \(\psi\).
 * For any \(v \in \mathbb{N} \setminus \{0\}\), the well-foundedness of \(<\) restricted to the recursive subset \(\{x \in OT \mid x < D_0D_{v+1}0\}\) in the sense of the non-existence of a primitive recursive infinite descending sequence is not provable under \(\textrm{ID}_v\).
 * The well-foundedness of \(<\) restricted to the recursive subset \(\{x \in OT \mid x < D_0D_{\omega}0\}\) in the same sense above is not provable under \(\Pi_1^1-\textrm{CA}_0\).

Extension
We introduce the extension of Buchholz's definition by Googology WIki user Denis Maksudov as follows :


 * \(C_\nu^0(\alpha) = \{\beta|\beta<\Omega_\nu\}\),
 * \(C_\nu^{n+1}(\alpha) = \{\beta+\gamma,\psi_\mu(\eta)|\mu,\beta, \gamma,\eta\in C_{\nu}^n(\alpha)\wedge\eta<\alpha\}\),
 * \(C_\nu(\alpha) = \bigcup_{n < \omega} C_\nu^n (\alpha)\),
 * \(\psi_\nu(\alpha) = \min\{\gamma | \gamma \not\in C_\nu(\alpha)\}\),

where

\(\Omega_\nu=\left\{\begin{array}{lcr} 1\text{ if }\nu=0 \\ \text{smallest ordinal with cardinality }\aleph_\nu \text{ if }\nu>0 \\ \end{array}\right.\)

There is only one little detail difference with original Buchholz definition: ordinal \(\mu\) is not limited by \(\omega\), now ordinal \(\mu\) belongs to previous set \(C_n\).

For example if \(C_0^0(1)=\{0\}\) then \(C_0^1(1)=\{0,\psi_0(0)=1\}\) and \(C_0^2(1)=\{0,...,\psi_1(0)=\Omega\}\) and \(C_0^3(1)=\{0,...,\psi_\Omega(0)=\Omega_\Omega\}\) and so on.

Limit of this notation must be omega fixed point \(\psi_0(\Omega_{\Omega_{\Omega_{\cdots}}})=\psi_0(\psi_{\psi_{\cdots}(0)}(0)) = \psi_0(\Lambda)\) (see the next section).

Normal form
The normal form for 0 is 0. If \(\alpha\) is a nonzero ordinal number \(\alpha<\Lambda=\text{min}\{\beta|\psi_\beta(0)=\beta\}\) then the normal form for \(\alpha\) is \(\alpha=\psi_{\nu_1}(\beta_1)+\psi_{\nu_2}(\beta_2)+\cdots+\psi_{\nu_k}(\beta_k)\) where \(k\) is a positive integer and \(\psi_{\nu_1}(\beta_1)\geq\psi_{\nu_2}(\beta_2)\geq\cdots\geq\psi_{\nu_k}(\beta_k)\) and each \(\nu_i\), \(\beta_i\) are ordinals satisfying \(\beta_i \in C_{\nu_i}(\beta_i)\) also written in normal form. More precisely, the normality of an expression of an ordinal can be described in a recursive way with respect to the corresponding ordinal notation system extending the original ordinal notation system \((OT,<)\) explained above.

Fundamental sequences
The fundamental sequence for an ordinal number \(\alpha\) with cofinality \(\text{cof}(\alpha)=\beta\) is a strictly increasing sequence \((\alpha[\eta])_{\eta<\beta}\) with length \(\beta\) and with limit \(\alpha\), where \(\alpha[\eta]\) is the \(\eta\)-th element of this sequence. If \(\alpha\) is a successor ordinal then \(\text{cof}(\alpha)=1\) and the fundamental sequence has only one element \(\alpha[0]=\alpha-1\). If \(\alpha\) is a limit ordinal then \(\text{cof}(\alpha)\in\{\omega\}\cup\{\Omega_{\mu+1}|\mu\geq 0\}\).

Although a system of fundamental sequences is not unique, there is a canonical choice of fundamental sequences in this community given by Denis. For nonzero ordinals \(\alpha<\Lambda\), written in normal form, fundamental sequences are defined as follows:


 * 1) If \(\alpha=\psi_{\nu_1}(\beta_1)+\psi_{\nu_2}(\beta_2)+\cdots+\psi_{\nu_k}(\beta_k)\) where \(k\geq2\) then \(\text{cof}(\alpha)=\text{cof}(\psi_{\nu_k}(\beta_k))\) and \(\alpha[\eta]=\psi_{\nu_1}(\beta_1)+\cdots+\psi_{\nu_{k-1}}(\beta_{k-1})+(\psi_{\nu_k}(\beta_k)[\eta])\),
 * 2) If \(\alpha=\psi_{0}(0)=1\), then \(\text{cof}(\alpha)=1\) and \(\alpha[0]=0\),
 * 3) If \(\alpha=\psi_{\nu+1}(0)\), then \(\text{cof}(\alpha)=\Omega_{\nu+1}\) and \(\alpha[\eta]=\Omega_{\nu+1}[\eta]=\eta\),
 * 4) If \(\alpha=\psi_{\nu}(0)\) and \(\text{cof}(\nu)\in\{\omega\}\cup\{\Omega_{\mu+1}|\mu\geq 0\}\), then \(\text{cof}(\alpha)=\text{cof}(\nu)\) and \(\alpha[\eta]=\psi_{\nu[\eta]}(0)=\Omega_{\nu[\eta]}\),
 * 5) If \(\alpha=\psi_{\nu}(\beta+1)\) then \(\text{cof}(\alpha)=\omega\) and \(\alpha[\eta]=\psi_{\nu}(\beta)\cdot \eta\) (and note: \(\psi_\nu(0)=\Omega_\nu\)),
 * 6) If \(\alpha=\psi_{\nu}(\beta)\) and \(\text{cof}(\beta)\in\{\omega\}\cup\{\Omega_{\mu+1}|\mu<\nu\}\) then \(\text{cof}(\alpha)=\text{cof}(\beta)\) and \(\alpha[\eta]=\psi_{\nu}(\beta[\eta])\),
 * 7) If \(\alpha=\psi_{\nu}(\beta)\) and \(\text{cof}(\beta) = \Omega_{\mu+1}\) for a \(\mu\geq\nu\) then \(\text{cof}(\alpha)=\omega\) and \(\alpha[\eta]=\psi_{\nu}(\beta[\gamma[\eta]])\) where \(\left\{\begin{array}{lcr} \gamma[0]=\Omega_\mu \\ \gamma[\eta+1]=\psi_\mu(\beta[\gamma[\eta]])\\ \end{array}\right.\).

It is an extension of the system of fundamental sequences up to \(\psi_0(\varepsilon_{\Omega_{\omega}+1})\) in Buchholz hierarchy given by modifying the rule ([].5) (ii) in recursive definition of the \(\textrm{dom}\) function in Buchholz's original paper by the rule 6 in the definition of \([]\) in p.6 in Buchholz's unpublished work.

If \(\alpha=\Lambda\) then \(\text{cof}(\alpha)=\omega\) and \(\alpha[0]=0\) and \(\alpha[\eta+1]=\psi_{\alpha[\eta]}(0)=\Omega_{\alpha[\eta]}\).

For comparison of ordinals written in normal form use the following property:

if \(\alpha<\beta\) and \(1\le\eta<\omega\) then \(\left\{\begin{array}{lcr} 0<\psi_\alpha(\gamma)\cdot\eta <\psi_\beta(\delta)\\ 0<\psi_\gamma(\alpha)\cdot\eta<\psi_\gamma(\beta)\\ \end{array}\right.\)

The comparison of expressions of ordinals can be described in a recursive way with respect to the corresponding ordinal notation system extending the original ordinal notation system \(OT\). In particular, the system of fundamental sequences above induce a recursive system of fundamental sequences on the corresponding ordinal notation, and hence the fast-growing hierarchy associated to it is recursive.

Common Mistakes
There are many common mistakes by beginners: It is probably because Buchholz's function is one of the simplest ordinal collapsing functions, and hence beginners tend to read introductory articles on Buchholz's function before understanding more elementary notions such as functions and ordinals. Also, beginners who do not understand Buchholz's function tend to recommend other beginners to learn ill-defind functions being unaware of the ill-definedness, and it is one of the main reasons why they mix up Buchholz's function with other ill-defined functions.
 * 1) Buchholz's function is not a computable function, while beginners say something like "Buchholz's function is obviously computable!" This is because there are uncountably many ordinals, and it's impossible to compute functions with uncountable domains since you'd need infinite time to read the whole input.
 * 2) The ordinal \(\psi_{\nu}(\alpha)\) with respect to Buchholz's function is defined for all \((\nu,\alpha) \in (\omega+1) \times \textrm{On}\), while beginners say something like "\(\psi_0(\Omega_{\omega}+1)\) with respect to Buchholz's function is obviously ill-defined!"
 * 3) Buchholz's function is neither a restriction of extended Buchholz's function nor extended Buchholz's function itself, while beginners say something like "\(\psi_0(\alpha)\) with respect to Buchholz's function obviously coincides with \(\psi_0(\alpha)\) with respect to extended Buchholz's function for any ordinal \(\alpha\)!"
 * 4) The ordinal \(\psi_0(\varepsilon_0+1)\) with respect to Buchholz's function coincides with \(\varepsilon_0\), while beginners say something like "\(\psi_0(\varepsilon_0+1)\) with respect to Buchholz's function obviously coincides with \(\varepsilon_0 \times \omega\)!"
 * 5) The ordinal \(\psi_0(\psi_1(\psi_2(\psi_3(0))))\) with respect to Buchholz's function coincides with \(\psi_0(\psi_2(0))\) with respect to Buchholz's function, while beginners say something like "\(\psi_0(\psi_1(\psi_2(\psi_3(0))))\) with respect to Buchholz's function obviously coincides with \(\psi_0(\psi_3(0))\) with respect to Buchholz's function!" This is partially due to the fact that \(\psi_1(\psi_2(\psi_3(0)))\notin C_0(\psi_1(\psi_2(\psi_3(0))))\).
 * 6) The sequence \(\psi_0(0), \psi_0(\psi_1(0)), \psi_0(\psi_1(\psi_2(0))), \psi_0(\psi_1(\psi_2(\psi_3(0)))), \ldots\) with respect to Buchholz's function is an eventually constant sequence with limit \(\psi_0(\psi_2(0))\) with respect to Buchholz's function, while beginners say something like "The limit of \(\psi_0(\psi_1(\psi_2(\psi_3(\cdots))))\) with respect to Buchholz's function obviously coincides with \(\psi_0(\psi_{\omega}(0))\) with respect to Buchholz's function!"
 * 7) The ordinal \(\psi_I(0)\) with respect to extended Buchholz's function coincides with \(I\), while beginners say "\(\psi_I(0)\) with respect to extended Buchholz's function obviously coincides with the least omega fixed point!" See also the issue on the \(\psi_0(\psi_I(0))\).