# Function $f:\mathbb R^+\rightarrow \mathbb R^+$ that is eventually greater than $x^{x^{x^{…^{x^x}}}}$

For each $n$, define $f_n:\mathbb R^+\rightarrow \mathbb R^+$ by $f_n(x) = \underbrace{x^{x^{x^{...^{x^x}}}}}_n$

I want to find a function $f:\mathbb R^+\rightarrow \mathbb R^+$ such that for any given $n$, $f$ is eventually greater than $f_n$.

Here $\mathbb R^+$ means the non-negative reals.

• Note that $\underbrace{x^{x^{x^\dots}}}_n=^n\!\!x$ – JSCB Sep 5 '12 at 13:10
• see also en.wikipedia.org/wiki/Tetration and note that andres answer is like tetration of a number with itself, kind of like how squaring is multiplication of a number with itself – binn Sep 5 '12 at 13:14

To make notation smoother, write $f(n,x)$ for $f_n(x)$. Let $$f(x)=f(\lceil x\rceil, x).$$ Here $\lceil x\rceil$ is the "ceiling" function that gives the smallest integer $\ge x$.
• I don't follow, what is $f(x,x)$? – fretty Sep 5 '12 at 12:56
• @AdamRubinson: It does not depend on $n$. It is given explicitly in terms of $x$. – André Nicolas Sep 5 '12 at 13:01
• @AdamRubinson: No, the function does not depend on $n$. (It is defined in terms of the $f_n$s, but the function itself is independent of $n$. For instance, for $10 \le x \le 11$, we have $g(x) = f_{10}(x)$, and for $20 \le x \le 21$, we have $g(x) = f_{20}(x)$, etc. But the function $g$ does not depend on $n$.) – ShreevatsaR Sep 5 '12 at 13:03
• @AdamRubinson: Here is a similar problem: find a function that eventually grows faster than any $x^n$. Solution: Let $g(x)=x^{\lceil x\rceil}$. Exactly the same idea. – André Nicolas Sep 5 '12 at 13:08
• @AdamRubinson: To solve the problem of majorizing the $x^n$, the familiar $e^x$ will do. I was just using the $x^{\lceil x\rceil}$ to illustrate the idea of the main proof in a simpler setting. – André Nicolas Sep 5 '12 at 16:21