# I Odd/even functions and fractional indices

#### dyn

Hi.
I would like to check that my understanding is correct. For $f(x)=x^{1/n}$ where n is an integer. If n is odd then f(x) is an odd function while if n is even then f(x) is neither odd or even as it involves the square root function which is only defined for non-negative x.
For $f(x) = x^{m/n}$ where n and m are integers then the above rule applies and if m is even then f(x) is even and m odd gives f(x) odd.
Examples $f(x) = x^{1/4}$ and $f(x) = x^{3/2}$ are neither odd or even as they are only defined for non-negative x and $f(x) = x^{2/5}$ is even and $f(x) = x^{3/5}$ is odd.
Have i got all this right ?
Thanks

#### mfb

Mentor
How do you define any of these for negative x? There are ways to define xc for all complex x and c but that won't work nicely in the real numbers, and the result will not be an odd or even function.

#### Stephen Tashi

Hi.
I would like to check that my understanding is correct. For $f(x)=x^{1/n}$ where n is an integer. If n is odd then f(x) is an odd function while if n is even then f(x) is neither odd or even as it involves the square root function which is only defined for non-negative x.
It's hard to formulate a systematic and rigorous treatment of fractional exponents when restricted to the real number system.

In the complex number system, the notation $x^{1/n}$ is ambigous. A non-zero complex number has n distinct n-th roots. (For example, 1 has 3 distinct cube roots.) So the notation $f(x) = x^{1/n}$ does not define a function.

Restricting ourselves to the real number system, a high school algebra course might expect students to evaluate $(-8)^{2/3}$ as $)(-8)^{1/3})^2 =(-2)^2 = 4$. With such a convention, the odd roots of numbers are unique. However we have the problem that $(-8)^{4/6}$ is not defined for $x < 0$ but after we reduce the fraction 4/6 to 2/3, we get something that is defined.

The rule $(x^a)^b = x^{ab}$ does always work when $a$ and $b$ are not integers.
[The above sentence should be corrected to say "does not always work"]
For example , let $x = -1,\ a = 2/3,\ b = 3/2$. With the above convention $((-1)^{2/3}))^{3/2} = (1)^{3/2} = 1$ But $(-1)^{(2/3)(3/2)} = (-1)^1 = -1$.

The high school algebra texts of my youth did not present an exhaustive treatment of fractional exponents. The authors knew that fraction exponents are a minefield when restricted to the real number system. The implied advice was "Don't waste your time pondering fractional exponents until you study complex numbers". Perhaps that's good advice!

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• RPinPA and DaveE

#### dyn

In my original question I was only referring to real numbers . That's why I stated $x^{1/2}$ is neither odd or even because it only exists for non-negative numbers while the graph of $x^{1/3}$ shows that it is an odd function

#### Stephen Tashi

where n and m are integers then the above rule applies and if m is even then f(x) is even and m odd gives f(x) odd.
The set of "integers" includes the negative integers.

#### Stephen Tashi

Have i got all this right ?
Yes, in the sense that people can find a way of interpreting your notation and reach the same conclusions. What is lacking is your own explanation of your notation.

In normal mathematical notation, if $a = b$ and $x^a$ exists then $x^b$ exists and $x^a = x^b$. However, apparently in you notation $4/6 = 2/3$ and $(-8)^{2/3}$ exists, but $(-8)^{4/6}$ does not. So your conclusions cannot be explained by properties of numbers alone. Your notation indicates particular algorithms. To deduce your conclusions we must define how notation is interpreted as algorithms. Someone who wished to define $x^{m/n}$ by an algorithm that reduced $m/n$ to its "lowest terms" before proceeding would disagree with your conclusions.

#### Quasimodo

The rule $(x^a)^b = x^{ab}$ does always work when $a$ and $b$ are not integers.
[The above sentence should be corrected to say "does not always work"]
For example , let $x = -1,\ a = 2/3,\ b = 3/2$. With the above convention $((-1)^{2/3}))^{3/2} = (1)^{3/2} = 1$ But $(-1)^{(2/3)(3/2)} = (-1)^1 = -1$.
1^(3/2)=-1, as well!

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#### mfb

Mentor
In my original question I was only referring to real numbers . That's why I stated $x^{1/2}$ is neither odd or even because it only exists for non-negative numbers while the graph of $x^{1/3}$ shows that it is an odd function
$x^{1/3}$ doesn't have an unambiguous value for negative x. You can define it to be $-(-x)^{1/3}$ but that is not the only option, and it leads to trouble when you want to look at e.g. $x^{2/6}$. Which is the same, as 1/3 = 2/6, but now you could evaluate it completely differently.

#### dyn

The rule $(x^a)^b = x^{ab}$ does always work when $a$ and $b$ are not integers.
For example , let $x = -1,\ a = 2/3,\ b = 3/2$. With the above convention $((-1)^{2/3}))^{3/2} = (1)^{3/2} = 1$ But $(-1)^{(2/3)(3/2)} = (-1)^1 = -1$.
Do you mean the rule $(x^a)^b = x^{ab}$does NOT always work when $a$ and $b$ are not integers ? I can only find that $(x^a)^b = x^{ab}$ without much reference to what $a$ and $b$ are

#### Stephen Tashi

Do you mean the rule $(x^a)^b = x^{ab}$does NOT always work when $a$ and $b$ are not integers ?
Yes, that's what I meant.

I can only find that $(x^a)^b = x^{ab}$ without much reference to what $a$ and $b$ are
There are many souces on the web state "the laws of exponents" without stating the proper restrictions that must be applied to the laws. For example, the Wolfram page http://mathworld.wolfram.com/ExponentLaws.html hints that the formula on it apply only to integral exponents by using $m$ and $n$ to denote the exponents, but it doesn't state the restriction explicitly.

#### dyn

So if I encounter $(x^{m/n})^{p/q}$ in general terms without knowing $x,m.n,p,q$ what do I do ? Is it only defined for certain $x,m,n,p,q$ ?
In the past i'm sure I used $(x^{m/n})^{p/q} = x^{(mp)/(nq)}$ and it always seemed to work

#### Stephen Tashi

So if I encounter $(x^{m/n})^{p/q}$ in general terms without knowing $x,m.n,p,q$ what do I do ? Is it only defined for certain $x,m,n,p,q$ ?
In the past i'm sure I used $(x^{m/n})^{p/q} = x^{(mp)/(nq)}$ and it always seemed to work
My rule of thumb is that if $x \ge 0$ the laws of exponents for integer exponents provide patterns that work for rational exponents.

There might be some expert who knows a set of rules for manipulating fractional exponents that works automatically when the base is negative. That expert isn't me! If I encountered $((-8)^{m/n})^{p/q}$ in appyling math to something, I'd think about what the expression means in the particular application and ask myself if writing it as $(-8)^{(mp)/(nq)}$ meant the same thing.

#### Mark44

Mentor
1^(3/2)=-1, as well!
No.
Possibly you are thinking of this as $(1^{1/2})^3$, and mistakenly thinking that $1^{1/2} = \pm 1$.
$1^{1/2} = \sqrt 1$ which is unambiguously 1.

#### Quasimodo

No.
Possibly you are thinking of this as $(1^{1/2})^3$, and mistakenly thinking that $1^{1/2} = \pm 1$.
$1^{1/2} = \sqrt 1$ which is unambiguously 1.
No, I am thinking 1^(1/2)=x, implying x^2=1, etc.

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#### Mark44

Mentor
No, I am thinking 1^(1/2)=x, implying x^2=1, etc.
But the equations $x = 1^{1/2} = \sqrt 1$ and $x^2 = 1$ are not equivalent. In the first equation, x = 1 only, while in the second equation, $x = \pm 1$.

This is an error a lot of people make, thinking that, for example, $\sqrt 1 = \pm 1$.

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Never mind.

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#### dyn

This is an error a lot of people make, thinking that, for example, $\sqrt 1 = \pm 1$.
Surely the square root of +1 is +1 or -1 as both values squared give 1 ? Unless you are taking the square root sign to mean taking only the positive value ?

#### Mark44

Mentor
Surely the square root of +1 is +1 or -1 as both values squared give 1 ?
No. When we say "square root of a number," what we mean is the principal square root of that number.
Unless you are taking the square root sign to mean taking only the positive value ?
Yes, the principal square root is the positive square root. A given positive number n will have two square roots. The notation $\sqrt n$ is the positive number a such that $a^2 = n$.

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#### dyn

Hi.
I would like to check that my understanding is correct. For $f(x)=x^{1/n}$ where n is an integer. If n is odd then f(x) is an odd function while if n is even then f(x) is neither odd or even as it involves the square root function which is only defined for non-negative x.
For $f(x) = x^{m/n}$ where n and m are integers then the above rule applies and if m is even then f(x) is even and m odd gives f(x) odd.
Examples $f(x) = x^{1/4}$ and $f(x) = x^{3/2}$ are neither odd or even as they are only defined for non-negative x and $f(x) = x^{2/5}$ is even and $f(x) = x^{3/5}$ is odd.
Have i got all this right ?
Thanks
How do you define any of these for negative x? There are ways to define xc for all complex x and c but that won't work nicely in the real numbers, and the result will not be an odd or even function.
$x^{1/3}$ doesn't have an unambiguous value for negative x. You can define it to be $-(-x)^{1/3}$ but that is not the only option, and it leads to trouble when you want to look at e.g. $x^{2/6}$. Which is the same, as 1/3 = 2/6, but now you could evaluate it completely differently.
I am using Thomas' Calculus and it shows $x^{1/2}$ and $x^{3/2}$ as only existing for non-negative $x$ so they are neither odd or even functions. It also shows graphs showing that $x^{1/3}$ is an odd function and $x^{2/3}$ is an even function. It also states that the cube root function is defined for all real $x$ and that $x^{2/3} = (x^{1/3})^2$

#### Stephen Tashi

It looks like you have a typo there.

I'm rather sure you meant that to say:

The rule $(x^a)^b = x^{ab}$ does not always work when $a$ and $b$ are not integers.
That was acknowledged in post #10

#### Quasimodo

No. When we say "square root of a number," what we mean is the principal square root of that number.
NO! We certainly don't mean that!
Why don't you just admit that Stephen Tashi has made a mistake and come to his defence with such nonsense? The n-th roots of unity are known since 1600's.

#### Mark44

Mentor
No. When we say "square root of a number," what we mean is the principal square root of that number.
NO! We certainly don't mean that!
You took me out of context here. My reply, quoted above, was a response to what dyn wrote in post #17, where he said that $\sqrt 1 = \pm 1$.
Quasimodo said:
Why don't you just admit that Stephen Tashi has made a mistake and come to his defence with such nonsense? The n-th roots of unity are known since 1600's.
Sure, but that's not what is being discussed here. The notation $\sqrt n$ or $\sqrt n$ signifies the principal square root or fourth root of a nonnegative real number n. The fact that n has two square roots and four fourth roots is not relevant in this discussion.

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#### Quasimodo

Sure, but that's not what is being discussed here. The notation √nn\sqrt n or 4√nn4\sqrt n signifies the principal square root or fourth root of a nonnegative real number n. The fact that n has two square roots and four fourth roots is not relevant in this discussion.
If it is a matter of notation, refer me where it is written that $1^{1/2} =1$ as Mr. Tashi claims. (Because, I do not see any square root symbol...

#### Mark44

Mentor
If it is a matter of notation, refer me where it is written that $1^{1/2} =1$ as Mr. Tashi claims. (Because, I do not see any square root symbol...
Check any precalculus textbook and it will show that $x^{1/2}$ and $\sqrt x$, with $x \ge 0$ are different notations that mean the same thing

#### dyn

I am using Thomas' Calculus and it shows $x^{1/2}$ and $x^{3/2}$ as only existing for non-negative $x$ so they are neither odd or even functions. It also shows graphs showing that $x^{1/3}$ is an odd function and $x^{2/3}$ is an even function. It also states that the cube root function is defined for all real $x$ and that $x^{2/3} = (x^{1/3})^2$
Any thoughts on the above found in Thomas' Calculus ? If $x^{1/3}$ is defined for all real $x$ then surely $x^{2/6}$ must be equivalent to it as 2/6=1/3 or we are in serious trouble !
$x^{1/3}$ doesn't have an unambiguous value for negative x. You can define it to be $-(-x)^{1/3}$ but that is not the only option, and it leads to trouble when you want to look at e.g. $x^{2/6}$. Which is the same, as 1/3 = 2/6, but now you could evaluate it completely differently.