Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Analysis Question -

  1. Feb 11, 2010 #1
    1. The problem statement, all variables and given/known data
    Suppose f:[a,b]->R is continuous and that f([a,b]) is a subset of Q (rational numbers). Prove that f is constant on [a,b]

    2. Relevant equations

    3. The attempt at a solution
    The solution states that:

    f([a,b]) must contain only one point, because if it has more than that, then it would have to be an irrational point. Which is a contradiction.

    My questions are, why does f([a,b]) contain only one rational point? Why can't it be a collection of rational points?

    Thank you for your help.

  2. jcsd
  3. Feb 11, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Because then f would be discontinuous! (assuming the statement is true, which it is)
  4. Feb 11, 2010 #3
    Remember that between any 2 rational points (in R) there is an irrational point.
  5. Feb 11, 2010 #4
    You guys are life savers.

    Thank you!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook