# What is $x$ if $x^{x^x} = {(1/2)}^{\sqrt 2}$?

What is $$x$$ if $$x^{x^x} = {(1/2)}^{\sqrt 2}$$?

Answer provided more or less like this : \begin{align}{\left(\frac 12\right)}^{\sqrt 2}&=\frac1{2^{\sqrt 2}}\\ 2^{\sqrt 2} &= 2^{(2^{(2/4)})} \\ &= 2^{(4^{(1/4)})}\\ & = 4^{1/2(4^{(1/4)})} \\ &= 4^{2^{-1}(4^{(1/4)})} \\ &= 4^{4^{-1/2}(4^{(1/4)})}\\ &= 4^{(4^{(-1/4)})}\\ &= 4^{({1/4}^{(1/4)})} \\ {\left(\frac 12\right)}^{\sqrt 2} &= \frac1{2^{\sqrt 2}} \\ &= \frac1{4^{({1/4}^{(1/4)})}} \\ &= {\left(\frac14\right)}^{({1/4}^{(1/4)})} \\ x&=\frac 14\end{align}

Is there more elegant way that show $$x=\frac 14$$ the only answer?

Edit : source https://youtu.be/d-E5isaIDTA

• well I think your way is a good one, since for another ways you have to you numerical algorithms which are not as elegant as yours
– user715522
Feb 5, 2020 at 16:17
• good solution, indeed. $\to +1$ Feb 5, 2020 at 16:35
• It's not the only answer, if you consider the complex plane. Using numerical methods you find x=2.3528266687 - i * 0.746689357999 works. Feb 5, 2020 at 16:35
• @skbmoore. There are so many solutions in the complex domain ! Feb 5, 2020 at 16:40
• The OP is almost certainly interested only in positive real values for $x$. But their approach only shows that $x=1/4$ is a solution, not that it's the only solution. I for one am hard pressed to think of a pre-calculus way of ruling out the existence of other (real) solutions. (Indeed, even a calculus-based proof looks like it'll be a little complicated.) Feb 5, 2020 at 16:47

## 1 Answer

Here is a calculus-based proof that $$x=1/4$$ is the only positive real solution to the equation $$x^{x^x}=(1/2)^\sqrt2$$.

It suffices to show that $$x^{x^x}$$ is increasing for all $$x\gt0$$. This is clear for $$x\ge1$$, so it remains to consider what happens for $$0\lt x\lt1$$. It's convenient to let $$x=e^{-u}$$ with $$u\gt0$$, and, after taking logarithms (twice), show that $$f(u)=\ln u-ue^{-u}$$ is increasing (from $$-\infty$$ as $$u\to0$$ to $$\infty$$ as $$u\to\infty$$). For this we need to show that

$$f'(u)={1\over u}-(1-u)e^{-u}={1-u(1-u)e^{-u}\over u}\ge0$$

for all $$u\gt0$$. But this is clear since $$u\gt0$$ implies $$e^{-u}\lt1$$, hence

$$u(1-u)e^{-u}\lt u(1-u)\le{1\over4}\lt1$$

(the maximum for $$u(1-u)$$ occurring at $$u=1/2$$).

If there is a non-calculus proof that $$x^{x^x}$$ is increasing, I'd be keen to see it.