Seems rational

Algebra Level 3

If there are four distinct numbers p p , q q , r r , and s s such that p q = r s \dfrac{p}{q} = \dfrac{r}{s} holds true, then is

x p q = x r s \large x^{\frac{p}{q}} = x^{\frac{r}{s}}

also true for all x C x \in \mathbb{C} ?

No Yes

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1 solution

Efren Medallo
Apr 11, 2017

Considering the fact that we are taking into consideration non-bijective relations in this problem, one must get accustomed in dealing with them.

A function f ( x ) = x 1 m f(x) = x^{\frac{1}{m}} is a one-to-many relation in C \mathbb{C} . That is, if x = 1 x = 1 , then there are m m values to f ( x ) f(x) , (essentially, the m t h m^{th} roots of unity). On the other hand, a function g ( x ) = x n g(x) = x^{n} , is a many-to-one relation in C \mathbb{C} .

When we resolve fractional exponents, note that we do not confine ourselves to the "obvious" solutions. Instead, we get a set of solutions.

For instance, why don't we try evaluating

( 4 ) 2 4 (-4)^{\frac{2}{4}}

Acknowledging that 2 4 = 1 2 \frac{2}{4} = \frac{1}{2} ,

( 4 ) 1 2 = { 2 i , 2 i } (-4)^{\frac{1}{2}} = \{ 2i, -2i \}

Well then, we can also do it this way:

( 4 ) 1 4 = { 1 + i , 1 i , 1 i , 1 + i } (-4)^{\frac{1}{4}} = \{ 1+i, 1-i, -1-i, -1+i \}

then we can square them and get

( ( 4 ) 1 4 ) 2 = { ( 1 + i ) 2 , ( 1 i ) 2 , ( 1 i ) 2 , ( 1 + i ) 2 } ( (-4)^{ \frac{1}{4}})^2 = \{ (1+i)^2, (1-i)^2, (-1-i)^2, (-1+i)^2 \}

= { 2 i , 2 i , 2 i , 2 i } = \{ 2i, -2i, 2i, -2i \}

= { 2 i , 2 i } = \{ 2i, -2i \}

But, note that we are not bound to solve it this way

( 4 ) 2 = 16 (-4)^2 = 16

( 16 ) 1 4 = { 2 , 2 , 2 i , 2 i } (16)^{\frac{1}{4}} = \{ 2, -2, 2i, -2i \}

as we will be getting extraneous values.

Extraneous solutions arise when we carry out a many-to-one, then one-to-many operations. Thus, the convention is to carry out the one-to-many operation first, then the many-to-one. Note that this will only apply when the fractional exponents have a common factor, i.e., it is not reduced to its lowest terms. In such case, both ways will yield the same answer.

@Efren Medallo your problem reminded me of
L = lim x 0 x 0 x L=\lim_{x \to 0} x^{\frac{0}{x}}

find L L when x is a complex no.

in that will the answer be 1

ABHIJIT DIXIT - 4 years ago

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