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Let $\star$ be any operation in the sequence of hyperoperations $(\text{Succ},+,\times,\uparrow,\uparrow\uparrow,\ldots)$, and consider the $\star$-factorial function defined as follows on the positive integers:

$$\begin{align}f_\star(n) &:= n\star(n-1)\star(n-2)\ldots \star 1\quad (n\text{ operands}) \end{align}$$

where a right-to-left order of evaluation is assumed; e.g. $f_\uparrow(3) = 3\uparrow 2\uparrow 1 = 3\uparrow (2\uparrow 1)$.

(With appropriate treatment for an argument of $n=0$, the definition could be extended to the nonnegative integers, but it would not be useful to do so in the present context.)

For each $\star$ operation, we want to compare the growth-rate of $f_\star$ to the following function $g_\star$: $$\begin{align}g_\star(n) &:= b\star' n\\ &= b\star b\star b\ldots \star b\quad (n+1\text{ operands})\quad\text{ if }\star =\text{Succ}\\ &= b\star b\star b\ldots \star b\quad (n\text{ operands})\quad\text{ if }\star \in(+,\times,\uparrow,\uparrow\uparrow,\ldots) \end{align}$$ where $\star'$ is the hyperoperation just after $\star$, and $b\ge 2$ is some given fixed integer. We use the concept of eventual domination; i.e., for two functions $f,g$ on the positive integers, the notation $f<^*g$ (resp. $f>^*g$) means that $f(n)<g(n)$ (resp. $f(n)>g(n)$) for all sufficiently large $n$.

Examples

$$\begin{align} f_\text{Succ}(n)&=n & \color{blue}{<^*} \ \ g_\text{Succ}(n)=b+n\quad\quad\quad\quad\text{ if }b\ge 2\\ f_+(n)&=\frac{n(n+1)}{2} & \color{red}{>^*} \ \ g_+(n)=b\times n\quad\quad\quad\quad\text{ if }b\ge 2\\ f_\times(n)&=n! & \color{red}{>^*} \ \ g_\times(n)=b\uparrow n\quad\quad\quad\quad\text{ if }b\ge 2\\ f_\uparrow(n)&=n^{(n - 1)^{(n - 2)^{\cdots^1}}} & \color{blue}{<^*} \ \ g_\uparrow(n)=b\uparrow\uparrow n\quad\quad\quad\quad\text{ if }b\ge 3 \end{align}$$

The first two examples above are easily worked out, while the third example is a well-known result for the standard factorial function. The fourth example follows from properties of the exponential factorial function proved by @Deedlit. (Note that in the fourth example, domination would occur in the reverse order if $b=2$; i.e., $f_\uparrow(n) \color{red}{>^*} 2\uparrow\uparrow n$.)

Question 1: Is it the case that if $b=3$ then $f_\star \color{blue}{<^*} g_\star$ for all $\star\in(\uparrow,\uparrow\uparrow,\ldots)$? I.e., in terms of Knuth's uparrows, do we have $f_{\uparrow^k}(.)\color{blue}{<^*}3\uparrow^{k+1}(.)$ for all $k\ge 1$?

Question 2: If Question 1 has a negative answer, then is it the case that for each $\star\in(\uparrow,\uparrow\uparrow,\ldots)$ there exists a $b$ (depending on $\star$) such that $f_\star \color{blue}{<^*} g_\star$? I.e., is there, for each $k\ge 1$, a $b_k$ such that $f_{\uparrow^k}(.)\color{blue}{<^*}b_k\uparrow^{k+1}(.)$?

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1 Answers1

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For convenience, we will let $a \uparrow^b 0 = 1$ and $a\uparrow^0 b = ab$.

As in the proof for the exponential case, we can prove that $3 \uparrow^{k+1} (n-2) < f_{\uparrow^k}(n) < 3 \uparrow^{k+1} (n-1)$ for $n \ge 2$. The left inequality is trivial, so we will prove the right hand side.

First, we need a lemma:

Lemma. $(n+1) \uparrow^k m > n\uparrow^k m + n$ for $n, k \ge 1, m\ge 2$.

Case $k = 1$: $(n+1)^m > n^m + m\cdot n^{m-1} > n^m +n$.

Inductive step:

$$(n+1)\uparrow^k m = (n+1) \uparrow^{k-1} [(n+1)\uparrow^k (m-1)] > (n+1)\uparrow^{k-1}(n \uparrow^k (m-1)) $$ $$> n \uparrow^{k-1}(n \uparrow^k (m-1)) + n \text{ (by inductive hypothesis)} = n \uparrow^k m + n$$

Now for the main proof:

Statement. $(n+2) \uparrow^k f_{\uparrow^k}(n) \le 3 \uparrow^{k+1} n$ for $n \ge 1$.

Base case: $3 \uparrow^k f_{\uparrow^k}(1) = 3 \uparrow^k 1 = 3 = 3\uparrow^{k+1} 1$.

Inductive step: $$ 3 \uparrow^{k+1} (n+1) = 3\uparrow^k (3\uparrow^{k+1} n)$$ $$ \ge 3\uparrow^k ((n+2) \uparrow^k f_{\uparrow^k}(n)) > 3 \uparrow^k ((n+1)\uparrow^k f_{\uparrow^k}(n) + n+1) \text{(by the lemma)} > (3\uparrow^k (n+1)) \uparrow^k [(n+1)\uparrow^k f_{\uparrow^k}(n)]$$ (by the Knuth Arrow theorem) $$> (n+3) \uparrow^k ((n+1) \uparrow^k f_{\uparrow^k}(n)) = (n+3) \uparrow^k f_{\uparrow^k}(n+1).$$

This proves the statement. But then,

$$3 \uparrow^{k+1}(n-1) \ge (n+1) \uparrow^k f_{\uparrow^k}(n-1) > n \uparrow^k f_{\uparrow^k}(n-1) = f_{\uparrow^k}(n).$$

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