Mathematical Meaning of Undefined
Date: 12/19/2002 at 13:33:31 From: Melissa Subject: Mathematical meaning of the word "undefined" My math teacher wants us to find out the meaning of the word "undefined" in mathematical terms.
Date: 12/19/2002 at 14:18:47 From: Doctor Ian Subject: Re: Mathematical meaning of the word "undefined" Hi Melissa, Something is 'undefined' when there is no good way to define it. The classic example is division by zero. There is no good definition for n / 0 = ? because any definition you try to put in there creates problems with other things in math. In this case, the problem is just that n / 0 = ? and n = 0 * ? are just two ways of saying the same thing. So if you decide that you can multiply something by zero and get something _other_ than zero, then you have to change the definition of zero, which would be a huge problem. So we say that division by zero is undefined. Another example would be the logarithm of a negative number. Logarithms are defined this way: a log x = a means that x = b b For example, 4 log 16 = 4 means that 16 = 2 2 Now, suppose x is negative instead of positive: ? log (-16) = ? means that -16 = 2 2 There is just no way to choose a value for '?' that makes sense: 2^x is positive when x is positive; and it's positive when x is negative; and it's positive when x is zero. So we've run out of possibilities. The thing you have to keep in mind, though, is that something that is undefined today may end up with a definition tomorrow. The classic example of that is the square root of a negative number: Complex Numbers http://mathforum.org/library/drmath/view/53877.html The square root of -1 used to be undefined. Now it's defined as the 'imaginary' number 'i', which lets us deal with the square root of any negative number: sqrt(-4) = sqrt(4 * -1) = sqrt(4) * sqrt(-1) = 2 * i Imaginary numbers also let us deal with logarithms of negative numbers: The Log of a Negative Number http://mathforum.org/library/drmath/view/55564.html However, they haven't helped us find a way to define division by zero. But that's not to say that someday, someone won't come up with one! Does this help? - Doctor Ian, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.