# Complex numbers and negative roots

1. Aug 10, 2015

### alvin51015

I was wondering if scientists or mathematicians have any use for complex numbers involving negative roots of I as in i=(-1)^(1/2). but my question is more what would be (-1)^(-1/2)?

2. Aug 10, 2015

### Staff: Mentor

Complex numbers are used everywhere. Making your computer would have been much harder without complex numbers in circuit design and quantum mechanics, for example.
Also $\pm i$, as $\frac{1}{i}=-i$ which you can see if you multiply both sides by i.

3. Aug 10, 2015

### Hector Mata

Yes, they are all over the place in math and physics. Indeed, there are entire branches of physics that would be impossible to do without them.

$$\sqrt{-1}$$

is represented by the letter i.

https://en.wikipedia.org/wiki/Complex_number

4. Aug 12, 2015

### HOI

By the way, there are problems involved with defining "$i= \sqrt{-1}$'. It can be shown that, in the complex numbers, all numbers have two square roots- so that notation is ambiguous. A better way to define the complex numbers is as pairs of real numbers, (a, b) with addition defined by (a, b)+ (c, d)= (a+ c, b+ d) and multiplication defined by (ab- cd, ad+ bc).

It can be show that this is a field with additive identity (0, 0) and multiplicative identity (1, 0). Further, the field of real numbers can be identified with the subfield (a, 0).
If we then represent multiplication of a real number, a, by a complex number, (b, c) with (a, 0)(b, c) as (ab,, ac), every complex number can be written in the form (a, b)= (a, 0)+ (0, b)= a(1, 0)+ b(0, 1). We have already agreed to represent the real number "1" by "(1, 0)". If we now agree to write $i= (0, 1)$ we have (a, b)= a+ bi.

5. Aug 17, 2015

### thelema418

I don't understand what you mean when you say this notation is ambiguous. The radical sign means principal square root.

6. Aug 17, 2015

### symbolipoint

Better to start with $i^2=-1$.

7. Aug 17, 2015

### thelema418

sorry, I dont understand the point you are making about ambiguity.

8. Aug 17, 2015

### symbolipoint

Exponentiate both sides to the $\frac{1}{2}$ power.
Plus or Minus Square Root of negative 1; no more ambiguity.

9. Aug 17, 2015

### thelema418

Could you write in greater detail: that is equally confusing. HOI said the notation is ambiguous, and I do not understand if you are also saying the notation is ambiguous. It seems to me you are implying that $i = \pm \sqrt{-1}$. Further, I do not understand why you would not use $x^2 = -1$, and then label the principal root $i$ such that $x_1 = i$ and $x_2 = -i$. There does not appear to be anything ambiguous about this process to me.

10. Aug 17, 2015

### symbolipoint

Then why you say it was ambiguous? You made the correct "implication".

11. Aug 17, 2015

### thelema418

HOI said it was ambiguous. I am asking for clarification about what makes it ambiguous to HOI or anyone who agrees that it is ambiguous.

I do not agree that $i = \pm \sqrt{-1}$.

12. Aug 17, 2015

### symbolipoint

I do not understand you. You also decided that i=sqrt(-1) was ambiguous, and then you found the correct meaning from i^2. The meaning of i is either ambiguous or it is not.

You are disagreeing with a very logically derived concept or fact.

13. Aug 17, 2015

### symbolipoint

thelema418,
Maybe I see the trouble. You MUST understand signed numbers.
Example, 5, the real number, integer number, 5.

(5)(5)=25. Positive number.
(-5)(-5)=25. Again, positive number.

Maybe that is the only idea you were missing.

14. Aug 17, 2015

### jbriggs444

When taking the square root of a positive real number, the radical sign means the principle square root -- the square root that is positive.

When taking the square root of a negative real number or of a complex number with a non-zero imaginary part there are two square roots. But neither is a positive real number. There is no way to write down a formula for $i$ using real number arguments that could not equally well be considered to yield $-i$. The labels $i$ and $-i$ are arbitrary in this sense and could be swapped without changing any mathematics.

I think that this is what HOI was trying to get at.

15. Aug 17, 2015

### symbolipoint

Excellent point, but a little extra work to not be confusing. What you mean is that we have this:
(-i)(-i)=(-1)(i)(-1)(i)
(-1)(-1)(i)(i)
(+1)*i^2
i^2
-1

16. Aug 18, 2015

### HallsofIvy

Staff Emeritus
thelema418 wrote "I don't understand what you mean when you say this notation is ambiguous. The radical sign means principal square root."
How do you define "principal square root" for complex numbers?

Symbolipoint wrote "Exponentiate both sides to the $\frac{1}{2}$ power.
Plus or Minus Square Root of negative 1; no more ambiguity."
Exponenting both sides still leaves it ambiguous. Writing "plus or minus" does NOT remove that ambiguity- the complex numbers are NOT an ordered field. We cannot unambiguously designate some as "positive" and others "negative".

And writing "-i= (-1)i" does not help since the whole question is about distinguishing unambiguously between i and -i.

17. Aug 19, 2015

### thelema418

Personally, I would use polar coordinates and cis notation. I see it like a piece of paper that has a tear down the non-postive real-axis (arbitrarily). As you move counterclockwise from the non-positive real axis, you eventually loop over to the backside of the paper. Continuing to rotate, you are eventually back where you began. The principal root is what occurs on the front page before you get back to where you started; it is contingent on the aforementioned choices.

Back to HOI, I do not understand how the argand coordinates resolves notational ambiguity. If $i = (0, 1)$ and $-i = (0, -1)$, I'm not certain what is different (in the context of ambiguity). Are you really distinguishing between $i$ and $-i$ any differently? Couldn't we just then say $i = (0, -1)$ and $-i = (0, 1)$? And how is this different than the algebra involved in $(-i)(-i) = (-1)(i)(-1)(i)$?

18. Aug 19, 2015

### jbriggs444

Before you could use polar coordinates, you would have to define which square root of -1 you are going to place above the real axis and which below. How do you know which one gets the label $i$ and which one gets the label $-i$?

19. Aug 19, 2015

### thelema418

There is no indicators of where to put 1 and -1 on the real y axis either. The geometry just seems to be part of an assumptive framework to me, much like rotating counterclockwise and locating the cut. You could really do anything.

This seems like a geometric concern - not a notational ambiguity which is what HOI spoke of. Notationally, $i \equiv \sqrt{-1}$ seems less ambiguous to me than the proposed $i^2 = -1$. And even saying $(0, 1) = i$ does not define whether the y axis is increasing or decreasing in the downward direction. HOI claimed this was better than the notational ambiguity with $i \equiv \sqrt{-1}$.

20. Aug 19, 2015

### jbriggs444

But -1 and 1 are distinguishable. -1 times -1 is equal to 1. 1 times 1 is not equal to -1. That is not the case for i and -i. The copy of the complex numbers that you get by interchanging the two is isomorphic to the original.