This Seems Like It Should Be True

Irrational Irrational = Irrational \large\text{Irrational}^{\text{Irrational}}= \text{Irrational}

Is the above equation always true for irrational numbers ? Each irrational number can be different.

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6 solutions

Isaac Buckley
Dec 30, 2015

I'll give the classic example.

Assume Irrational Irrational = Irrational \color{#3D99F6}{\text{Irrational}}^{\color{#20A900}{\text{Irrational}}}= \color{#D61F06}{\text{Irrational}} .

Given that 2 \sqrt{2} is irrational, then so is 2 2 \sqrt{2}^{\sqrt{2}} .

Given that 2 2 \sqrt{2}^{\sqrt{2}} is irrational, then so is ( 2 2 ) 2 = 2 2 = 2 \left(\sqrt{2}^{\sqrt{2}}\right)^{\sqrt{2}}=\sqrt{2}^2=2 .

Clearly 2 2 is not irrational so our original assumtion was false.

A variation on a theme....

Either x = 2 2 x = \sqrt{2}^{\sqrt{2}} is rational or irrational. If it is rational then x x is our counterexample. If it is irrational, then x 2 = 2 2 = 2 x^{\sqrt{2}} = \sqrt{2}^{2} = 2 is our counterexample.

So while we've established that the given assertion is false we're not actually sure whether x x or x 2 x^{\sqrt{2}} is a specific counterexample, (not that it matters as far as this question is concerned, but it would be interesting to know). As it turns out, by the Gelfond-Schneider theorem x x is transcendental, (and hence irrational).

Note that none of integers, rational or complex numbers are closed under exponentiation. Respective counterexamples are 2 1 = 1 2 , 2 1 2 = 2 2^{-1} = \dfrac{1}{2}, 2^{\frac{1}{2}} = \sqrt{2} and i i = e π 2 . \large i^{i} = e^{-\frac{\pi}{2}}. It is closed for Z + \mathbb{Z}^{+} though.

Brian Charlesworth - 5 years, 5 months ago

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Why are complex numbers not closed under exponentiation? Real numbers are also complex numbers with zero imaginary part.

William Nathanael Supriadi - 3 years, 5 months ago

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Good point. I guess I meant that complex numbers with non-zero imaginary components are not closed under exponentiation.

Brian Charlesworth - 3 years, 5 months ago

Technically, the number should be

2 2 2 { \sqrt { 2 } }^{ { \sqrt { 2 } }^{ \sqrt { 2 } } }

not

( 2 2 ) 2 { \left( { \sqrt { 2 } }^{ \sqrt { 2 } } \right) }^{ \sqrt { 2 } }

Arulx Z - 5 years, 5 months ago

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Can you please explain why? I completely disagree.

Isaac Buckley - 5 years, 5 months ago

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I'm a bit confused to be honest but it seems like you are right, as verified by wolfram alpha.

Arulx Z - 5 years, 5 months ago

Me too. Without the parentheses, we count it from the top.

William Nathanael Supriadi - 4 years, 8 months ago

As we assume (✓2)^(✓2) to be irrational, both are right

Joel Sleeba - 3 years ago

I can't understand

Debasri Das - 2 years, 2 months ago
Hamza A
Dec 30, 2015

e^ln(n)=n

where n is a natural number
since e is irrational so ln(n) is irrational

so the answer is false

This is wrong for n = 1 n = 1 , since ln 1 = 0 \ln 1 = 0 is rational.

Ivan Koswara - 5 years, 5 months ago

oops

i was just making an example for irrational numbers raised to irrational numbers being rational

thanks for pointing it out :)

Hamza A - 5 years, 5 months ago

I think it may be a little bit complicated to illustrate ln(n) is irrational emm.

Menelausq MqQuannsjxk - 1 year, 1 month ago
Kamaal Mehdi
Jan 3, 2016

When you know the e^i^pie equals -1 , you can answer this in seconds

i π i\pi is not irrational.

Ivan Koswara - 5 years, 5 months ago

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Why i π i\pi is not irrational?

John Michael Gogola - 5 years, 5 months ago

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Rational and irrational numbers are subsets of real numbers. i π i\pi is not a real number.

Ivan Koswara - 5 years, 5 months ago

Purely imaginary number with an irrational magnitude.

Jack Han - 4 years, 9 months ago

can we talk about rational and irrational numbers when we have an imaginary number included?

maryam kouram - 3 years, 11 months ago

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It's possible. Gaussian integers are complex numbers in the form a + b i a + bi where a , b a, b are integers. I imagine you can talk about "Gaussian rationals" that are in the form a + b i a + bi where a , b a, b are rationals. But the usual definitions of "rational number" and "irrational number" only apply for real numbers.

Ivan Koswara - 3 years, 11 months ago

Well, irrational numbers have to be real

Juan Eduardo Ynsil Alfaro - 2 years, 6 months ago
Griffin Macris
Jul 20, 2016

Here's a more interesting example: Let a rational non-integer y y be given. Set y = x x y = x^x for some real number x x . It is a bit difficult to prove, but x x is actually a transcendental number. This gives infinitely many counterexamples.

W ( x ) e W ( x ) = x W(x)e^{W(x)}=x for every x x . It's also proven that W ( x ) W(x) is irrational for every algebraic number. W ( x ) W(x) is the Lambert-W function and integers is an algebraic number

Patience Patience
May 11, 2016

not always

What do you want to tell?????????????????????????????????????????????

Debasri Das - 2 years, 2 months ago

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