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We're all familair with this beautiful proof whether or not an irrational number to an irrational power can be rational. It goes something like this:

Take $(\sqrt{2})^{\sqrt{2}}$

If it's rational, then you proved it, if it's irrational, take $((\sqrt{2})^{\sqrt{2}} ){^\sqrt{2}} = 2$ and you've proved it.

I'm wondering if you can raise $\pi$ or $e$ to a certain non-trivial real power to make it rational? And if not, where is the proof that it can't be done?

p.s. - I almost left out the real part, but then I realized that $e^{i\pi} = -1$.

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    $\begingroup$ I assume the word "non-trivial" in your question is meant to exclude $\pi^0 = 1$. $\endgroup$
    – Ilmari Karonen
    May 22, 2013 at 21:37
  • $\begingroup$ What am I missing? wolframalpha.com/input/?i=sqrt+2%5Esqrt+2%5Esqrt+2 $\endgroup$
    – Double AA
    May 22, 2013 at 23:47
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    $\begingroup$ @DoubleAA You're missing Brackets... wolframalpha.com/input/?i=%28sqrt+2%5Esqrt+2%29%5Esqrt+2 $\endgroup$
    – Arjun Bajaj
    May 23, 2013 at 0:01
  • $\begingroup$ @IlmariKaronen Indeed, I was trying to sound fancy, did I use it incorrectly? $\endgroup$
    – OmnipresentAbsence
    May 23, 2013 at 14:36
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    $\begingroup$ Well... not incorrectly, just imprecisely. "Trivial" is one of the vaguest, most subjective and hand-wavey terms in mathematics; even in specific areas where it does have a reasonably well agreed-upon meaning, that meaning depends entirely on context. Anyway, if you were trying to sound like a working mathematician, I'd say you succeeded. However, I'd also say that's not necessarily something to be proud of. :) $\endgroup$
    – Ilmari Karonen
    May 23, 2013 at 16:39

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Of course. Pick any positive rational $p$ and let $x=\log_\pi p$, then $\pi^x=p$.

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    $\begingroup$ Derp, I am dumb. +1 $\endgroup$
    – OmnipresentAbsence
    May 22, 2013 at 18:05
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    $\begingroup$ Don't feel bad. It's easy to forget these things from time to time. $\endgroup$
    – Asaf Karagila ♦
    May 22, 2013 at 18:06
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    $\begingroup$ @Patrick: How so? You can define the logarithm without exponentiation. Define $\ln x=\int_1^x\frac1tdt$, then define $\log_p x=\frac{\ln x}{\ln p}$. You can now prove that $p^{\log_p x}=x$. $\endgroup$
    – Asaf Karagila ♦
    May 22, 2013 at 18:22
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    $\begingroup$ A geometric way of "seeing" this: The map $f: x\mapsto \pi^x$ is continuous and strictly increasing, so its graph $y=\pi^x$ passes through a lot of points where just one of the two coordinates (we want $y$) is rational. $\endgroup$
    – Jeppe Stig Nielsen
    May 22, 2013 at 21:58
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    $\begingroup$ @Jeppe: Except from rare occasions, the geometric way is never my way of seeing things! Thank you making that point, it wouldn't have crossed my mind. But the argument would be clearer if you state it as $f$ being surjective onto $(0,\infty)$, and therefore it hits every positive rational number. $\endgroup$
    – Asaf Karagila ♦
    May 22, 2013 at 22:00

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