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Sets, tuples, etc

  1. Jan 22, 2014 #1
    1. The problem statement, all variables and given/known data
    So if a question asks you for a pair set, with some criteria, is it enough to just say S = {a,b} or do you need something extra? Also this is another question that is from my set theory class, if the question defines a function and to solve over the Real numbers, ex. 1-|x^2-1| = 0, I know it isn't as simple as it looks, I know we are supposed to give all the values of x satisfying this function in a set, but I think our professor is looking for some kind of proof, like using, "if a = b then |a| = |b|". So my question is, how do I integrate those if statements while solving for all values of x?
     
  2. jcsd
  3. Jan 22, 2014 #2

    haruspex

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    There is a simple constraint for elements of a set that does not apply to elements of a tuple.
    There are three ways I can of for dealing with modulus signs.
    1. Square
    In this example you can write (x2-1)2 = 1. Not useful here, though.
    2. Consider each combination of the binary choices separately.
    To illustrate that I need a more complicated example: 1-|x^2-1| = |x+3|
    The four cases to be considered correspond to x2>1 versus x2<1, and x>-3 versus x<-3. Some combinations may turn out to be impossible.
    3. Find all the critical values of x and divide the real line up into corresponding intervals. In my example, the critical values are x=-1, x=+1, x=-3, so the line is divided into four intervals.

    Generally, method 3 scales better than method 2 (which grows cases exponentially).
     
  4. Jan 23, 2014 #3
    Thanks, that helped, I figured out the solution set to all parts of that question.
     
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