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Fractional Exponents and Complex Roots


Date: 06/11/99 at 03:59:08
From: David Hanson
Subject: Complex roots

We are having a discussion about unique representations of complex 
roots. 

Does z^(a/b) = (z^a)^(1/b) or (z^1/b)^a, and does z^(1/d) represent 
the complex number with the smallest positive argument, or is it 
ambiguous notation for all the d roots?

For example, does [(-4)^2]^(1/4) = 16^(1/4) = the principal fourth 
root of 16, which is 2, but [(-4)^(1/4)]^2, where if you take the 
fourth root of -4 with the smallest positive argument and then square 
it, you get 2i?

Can you help? Thanks.
David Hanson


Date: 06/11/99 at 05:15:44
From: Doctor Mitteldorf
Subject: Re: Complex roots

Dear David,

Even simpler, you can take (-2), square it, find the square root, and 
end up with a different number than you started with.

My advice is to keep your eye on the ball. The conventions are there 
to serve you, not to confine your thinking, and besides, conventions 
in this area are murky. You and your students should be aware of this 
kind of ambiguity, and alerted that when you are trying to find 
specific solutions to equations, ambiguity may arise so you will need 
to examine all roots, then think about what makes sense in the context 
of the present problem.

- Doctor Mitteldorf, The Math Forum
  http://mathforum.org/dr.math/   


Date: 06/11/99 at 05:42:58
From: Doctor Pete
Subject: Re: Complex roots

Hi,

As you can see from your examples, it is generally incorrect to take 
"principal" values during intermediate steps in evaluation. Doing so 
is equivalent to losing solutions, and the result is that the normal 
axioms of associativity, commutativity, and cancellation may be 
violated. If we are to make any sense of

     z^(a/b)

then it is evident that one must be careful in saying what a "b-th" 
root of a complex number is. Such a root is not a number, but a set. 
It is simply not correct to say that

     4^(1/2) = 2

rather, when dealing with the complex numbers,

     4^(1/2) = {2, -2}.

Similarly,

     1^(1/3) = {1, e^(2*pi*i/3), e^(-2*pi*i/3)}.

When dealing with complex numbers, one must become accustomed to the 
idea that functions that are bijective (one-to-one and onto) in the 
reals are not necessarily so in the complex plane, and hence functions 
should not be thought of as mappings of a number to a number, but of a 
set to a set. In the cube root of unity example above, the function

     f(z) = z^(1/3)

is a one-to-three mapping, taking (nearly every) complex number z and 
mapping it to three distinct elements. Similarly, the inverse function

     f^(-1)(z) = z^3

is a three-to-one mapping, where for a given value of z^3, there are 
three complex numbers that map to z^3 (one of them being z itself).

So it is at once clear that z^(1/n) is not "ambiguous" in the sense 
that its value is not specified - what is *meant* by the symbol 
z^(1/n) is not "the" n-th root of z, but the "n" n-th roots of z. 
A set of values is no more ambiguous than a single value.

To demonstrate how this changes your example, consider

     (-4)^(2/4).

Apply the axiom of cancellation to obtain

     (-4)^(1/2) = {2i, -2i}.

This is the correct solution set. On the other hand, one could do it 
this way:

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

so

     [(-4)^(1/4)]^2 = {(1+i)^2, (-1+i)^2, (-1-i)^2, (1-i)^2}
                    = {2i, -2i, 2i, -2i}
                    = {2i, -2i}.

But why isn't

     [(-4)^2]^(1/4) = (16)^(1/4) = {2, 2i, -2, -2i}

valid? This is for the same reason that

     -1 = i^2 = (-1)^(1/2)(-1)^(1/2) = [(-1)(-1)]^(1/2) = 1^(1/2) = 1

is invalid. What's going on is that we're mapping 2-to-1, then 1-to-2, 
creating extraneous solutions. Since this creates contradictions such 
as the above, it follows that whenever possible, we first evaluate 
one-to-many, then many-to-one. So the answer to your original question 
is that z^(a/b) indicates [z^(1/b)]^a. But this consideration arises 
only in the case where a and b have a common factor.

- Doctor Pete, The Math Forum
  http://mathforum.org/dr.math/   
    
Associated Topics:
High School Imaginary/Complex Numbers

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