Integers and Complex Numbers
Date: 02/27/97 at 19:52:17 From: Brian D Subject: Numbers besides complex A few weeks ago my math teacher told the class that all numbers are contained by the real and imaginary sets of numbers. I couldn't help but be a little curious because this seemed like over-generalization. I checked in the math FAQ and came up with some info that was mostly way over my head, but I could see that there might be other sets of numbers, particularly hyper-reals and octonions. Do these sets exist outside complex numbers, or did I just get confused with all of the math jargon? If they exist inside complex numbers, what sets are they in? Could you also explain what these sets are in terms that an average math-loving high schooler could understand? Thanks for taking time to read this and providing the Dr. Math service.
Date: 02/28/97 at 09:33:06 From: Doctor Mitteldorf Subject: Re: Numbers besides complex Dear Brian, I think your instincts about this are right. All numbers are abstractions, and how abstract you get is limited only by your imagination (no pun intended). When people had the natural numbers, they could add and multiply but they couldn't subtract. By expanding to the integers, they were able to subtract. Thus the expression 2-3 has meaning: you just call it -1. But this would be a futile endeavor if at the same time you introduced a new problem: what does it mean to subtract (-1) from something? Do you have to invent a new kind of number ("neganegs") that you get if you subtract (-1) from 0? In fact, you don't: it's not obvious, but it's true that you can define 0 - (-1) to be +1, the regular old positive integer 1, and you'll still have a system that's just as logically self-consistent as the one you had before, but now it's more complete. Continue the story: people had the integers, so they could add, subtract and multiply, but they couldn't divide. What did it mean to divide 1 by 2? You could invent a new kind of number, called a fraction. But then you need a self-consistent rule for dividing fractions. It turns out that the system of integers and fractions is one in which you can add, subtract, multiply and divide. (You have to make an exception for dividing by zero - no way around that.) Now you can add, subtract, multiply and divide; but you want to solve polynomial equations. You want to be able to say that every equation you write down, (for example x^2-2x+1 = 0 or x^3 = 5) has a solution. You can't do it within the context of the rational numbers (that's what you call the integers + fractions.) How about if we add all the "in-between numbers"; we admit numbers like 1.4143235... and 3.1415926... into our number system. That would be the obvious next thing to do, but you find it doesn't work! There are still equations like x^2+1 = 0 that don't have any solutions. So now we come to the point: By admitting one more number called i into your system (and making it "independent" of the real numbers, so that 1+i is just 1+i, its own thing, not reducible to any other form), you've now got a closed system in which every polynomial equation has a solution. You don't get into a jam when you try to solve equations like x^2 + i = 0, or x^17 = 1 + 6i. The "complex numbers" form a convenient home for mathematicians who want to be sure that whenever they write down a meaningful equation, it has a meaningful solution. At this point, you might offer the questioner an exercise: what is the square root of i? Hint: suppose it has the form a+bi where a and b are regular old real numbers. Can you write equations that tell you what a and b have to be? Can you solve them? How can you check your answer? This doesn't have to be the end of the story. There are all kinds of other objects that can be constructed. They tend to get progressively less useful as they get more abstract - after all, what could compare in usefulness to the counting numbers 1, 2, 3...? But, on the other hand, they keep mathematicians entertained. And a lot of things that were once thought to be pure, useless mathematical imagination have turned out to be crucially useful in some branch of science. I'd venture to say that the whole branch of physics called quantum mechanics couldn't have been figured out without complex numbers. -Doctor Mitteldorf, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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