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Proving a function is one-to-one

  1. Nov 26, 2007 #1
    1. The problem statement, all variables and given/known data

    Prove that the function f: Q ---> R, f(a/b) = (2^a)(3^b) is one to one, assuming that gcd(a,b) = 1, that is the fraction a/b is reduced.

    2. Relevant equations



    3. The attempt at a solution

    I know f is one to one if it never maps 2 different elements to the same place, i.e. f(a) does not equal f(b) whenever a does not equal b.

    I have tried looking at how other problems are proved to be one to one and am attempting to do it in a similar way:

    Suppose f(a/b) = f(c/d)
    then (2^a)(3^b) = (2^c)(3^d)

    I'm not sure what to do though. I don't think I'm doing this right. Please help me with this problem.
     
  2. jcsd
  3. Nov 26, 2007 #2

    EnumaElish

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    You're on the right track. Now suppose a/b is not equal to c/d. Can (2^a)(3^b) = (2^c)(3^d) still be the case?
     
  4. Nov 26, 2007 #3
    Well if a/b does not equal c/d then the equality does not hold. I think that a/b must equal c/d but I'm not sure how to really write it formally as a proof. Also,I just am not sure that this problem is so simple though. What does gcd(a,b) = 1 have to do with the problem?

    Nevermind, I got it now. Thanks for your help!
     
    Last edited: Nov 27, 2007
  5. Nov 27, 2007 #4

    Office_Shredder

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    gcd(a,b)=1 iff the fraction is reduced (i.e. there is no common multiple of a and b that you can cancel).

    Note that 2 and 3 are primes, and the fundamental theorem of arithmetic tells us something about primes being multiplied together.

    As an aside, this is quite a nice way of showing the set of rationals is countable, as 2a and 3b are in fact integers
     
  6. Nov 27, 2007 #5

    Hurkyl

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    That's not well defined. For example:
    6 = f(1/1) = f(-1/-1) = 1/6
     
  7. Nov 27, 2007 #6

    Office_Shredder

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    That's a good point... usually for stuff like this it's assumed the denominator is positive I think
     
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