show that $$\sqrt{7}^{\sqrt{8}}>\sqrt{8}^{\sqrt{7}}$$

and I found $$LHs-RHS=0.017\cdots$$

I have post this interesting problem Prove $\left(\frac{2}{5}\right)^{\frac{2}{5}}<\ln{2}$

can someone suggest any other nice method? Thank you everyone.

  • 3
    $\begingroup$ It is true that $7^8>8^7$, since $(1+\frac{1}{7})^7<e<7$. Then we have, $\sqrt{7}^8>\sqrt{8}^7$. Does it imply that $\sqrt{7}^\sqrt{8}>\sqrt{8}^\sqrt{7}$. $\endgroup$ – mtm Jun 28 '13 at 9:26
  • $\begingroup$ No,How can have $\sqrt{7}^8>\sqrt{8}^7$? $\endgroup$ – math110 Jun 28 '13 at 9:31
  • $\begingroup$ $\sqrt{7}^{16}>\sqrt{8}^{14}$. So if we take $\frac{1}{2}$ power of both sides, we have $\sqrt{7}^{8}>\sqrt{8}^{7}$. Maybe i am wrong. $\endgroup$ – mtm Jun 28 '13 at 9:37
  • $\begingroup$ $\sqrt{7}^{8}>\sqrt{8}^{7}\Longrightarrow \sqrt{7}^{\sqrt{8}}>\sqrt{8}^{\sqrt{7}}$? I think is wrong. $\endgroup$ – math110 Jun 28 '13 at 10:58
  • $\begingroup$ @mtm Monotonicity argument like yours will not work here, no matter you take $(\ln t)/\sqrt{t}$ or $(\ln t)/t$. $\endgroup$ – Shuhao Cao Jun 28 '13 at 15:21

Note that $\sqrt{7}^{\sqrt{8}}\doteq15.673$ and $\sqrt{8}^{\sqrt{7}}\doteq15.656$, so these two are pretty close. Furthermore $\sqrt{7}<e<\sqrt{8}$, and the function $f(x):={\log x\over x}$ has a local maximum at $x:=e$. This excludes the use of monotonicity arguments. The following proof uses integer arithmetic instead.

To begin with we need a rational approximation to $\sqrt{7\over8}$ that is slightly larger than $\sqrt{7\over8}$. Using Mathematica (or continued fractions) one finds that $$7\cdot 31^2=6727<6728=8\cdot 29^2\ ,$$ which implies $$\sqrt{7}\cdot31<29\cdot\sqrt{8}\ .$$ Furthermore one computes $$8^{29}=15\>47425\>04910\>67253\>43623\>90528<15\>77753\>82034\>84580\>66150\>42743=7^{31}\ .$$ It follows that $$\bigl(8^{\sqrt{7}}\bigr)^{31}<\bigl(8^{29}\bigr)^{\sqrt{8}}<\bigl(7^{31}\bigr)^{\sqrt{8}}\ .$$ Now take the $62^{\rm th}$ root on both sides.

  • 1
    $\begingroup$ Very instructive! +1 $\endgroup$ – Markus Scheuer Nov 13 '14 at 7:58

The challenge here is to demonstrate the inequality in a way that can easily be checked by hand. Taking logs, the inequality to be proved is

$$\sqrt7\ln8 \lt \sqrt8\ln7$$

which can be written as

$$3\sqrt{1-{1\over8}}\ln2 \lt \ln(8-1) = 3\ln2+\ln(1-{1\over8})$$

so the inequality to prove is

$$-\ln\left(1-{1\over8}\right)\lt 3\left(1-\sqrt{1-{1\over8}}\right)\ln2$$

We'll use the Taylor series


to obtain

$$\begin{align} -\ln\left(1-{1\over8}\right) &\lt {1\over8}+{1\over2}\left({1\over8}\right)^2+{1\over3}\left({1\over8}\right)^3+{1\over3}\left({1\over8}\right)^4\\ &={1\over8}+{1\over2}\left({1\over8}\right)^2+{1\over3}\left({1\over8}\right)^3{1\over1-{1\over8}}\\ &={1\over8}+{1\over128}+{1\over21}{1\over64}\\ &\lt{1\over8}+{1\over128}+{1\over16}{1\over64}\\ &={128+8+1\over2^{10}}\\ &={137\over2^{10}} \end{align}$$

We'll use the fact that all the coefficients in the Taylor series


are positive to get

$$1-\sqrt{1-{1\over8}}\gt {1\over2}{1\over8}+{1\over8}\left({1\over8}\right)^2={33\over2^9}$$

Finally, we use the easily verified identity


and the alternating Taylor series


to get

$$\begin{align} \ln2&\gt 3\left({1\over4}-{1\over2}\left({1\over4}\right)^2+{1\over3}\left({1\over4}\right)^3-{1\over4}\left({1\over4}\right)^4\right)+{3\over125}\\ &={21\over32}+{1\over64}-{3\over1024}+{3\over125}\\ &\gt{21\over32}+{1\over64}-{3\over1024}+{3\over128}\\ &={21\cdot32+16-3+24\over1024}={709\over1024} \end{align}$$



so that


Consequently, the inequality to verify is


which is to say,


Eschewing calculators, note that $1063=1024+32+8-1$, so




so the desired inequality holds.

  • 1
    $\begingroup$ Reading along, I kept expecting the computation to end, but it never did! A rather intimidating verification. $\endgroup$ – Ryan Reich Aug 8 '13 at 5:29
  • 3
    $\begingroup$ @RyanReich, but it did end! ;-) $\endgroup$ – Barry Cipra Aug 8 '13 at 6:00

I just want to give you a bigger picture on this question. The fact you point out corresponds to $f(2)>0.$

$f(\text{x$\_$})\text{:=}\left(t=\lfloor \exp (x)\rfloor ;a=t^{1/x};b=(t+1)^{1/x};a^b-b^a\right);$


enter image description here


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