The **bop-counting function** is a function defined by Harvey Friedman, arising in study of binary operations on structures.^{[1]}

## DefinitionEdit

Let \(D\) be a set. A *ternary relation on \(D\)* is a set \(R\subseteq D^3\). We call two ternary relations \(R \subseteq D^3,\, R' \subseteq D'^3\) *equivalent* or *isomorphic* iff there is a bijection \(f:D\rightarrow D'\) such that \((a,b,c)\in R\Leftrightarrow (f(a),f(b),f(c))\in R'\).

A ternary relation \(R\) is a *binary operator* or *bop* iff every \((a,b)\in D^2\) uniquely determines \(c\in D\) such that \((a,b,c)\in R\) (so that \(R\) is a function \(D^2\rightarrow D\)). For \(k\in\Bbb N\) a *\(k\)-restriction* of \(R\) is a domain restriction of \(R\) to a set \(E^2\) such that \(|E|=k\). Two bops \(R_1,R_2\) are *\(k\)-similar* if for every \(k\)-restriction of \(R_1\) we can find \(k\)-restriction of \(R_2\) which is isomorphic to it, and vice versa. \(k\)-similarity is an equivalence relation, and this is easy to show.

Finally, define the **bop-counting function** \(\theta(k)\) for \(k\in\Bbb N\) to be the number of equivalence classes under \(k\)-similarity.

## PropertiesEdit

By counting the number of possible (up to isomorphism) \(k\)-restrictions of a set, it can be show that \(\theta(k)\) is a function which is at most triply exponential in \(k\) (i.e. it is bounded by \(2^{2^{2^{ck}}}\) for some constant \(c\)). Nevertheless, Friedman has shown that this function is not recursive. This presents algorithmic difficulty in finding values of this function.

A more striking result regards the specific value \(\theta(12)\). Friedman has shown that the statement "\(\theta(12)=N\)", for any natural number \(N\), cannot be proven in any of well-known strong formal theories, including ZFC + "there exists an I1 rank-into-rank cardinal" and NBG + "there exists a nontrivial elementary embedding from \(V\) into \(V\)" (assuming they are consistent).

We can however try to measure how "close" a theory is to providing exact value of the number, by asking for upper bounds which the theory can provide. To be precise, for theory \(T\), let \(\theta(12,T)\) be the least \(N\) such that \(T \vdash \theta(12) \leq N\). If \(T,T'\) are "standard" large cardinal extensions of ZFC (with term "standard" being intentionally left vague; Friedman defers clarification until a later paper), and both are consistent, then if \(T'\) proves the consistency of \(T\) then \(\theta(12,T') < \theta(12,T)\). This, in a way, shows how bad these theories are at estimating \(\theta(12)\), as even a barely stronger system asserting consistency already shows tighter bound.

## ReferencesEdit

- ↑ Impossible counting by Harvey M. Friedman