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A question involving self-composition.

  1. Jul 23, 2008 #1
    I can't for the life of me figure this one out all the way.

    Suppose f : R -> R is differentiable, and consider g(x) = f(f(x)). Show that if g is monotone decreasing, then g must be constant.

    Here's what I've done so far (I'd hesitate to call it "progress"):

    By the chain rule, g'(x) = f'(f(x)) f'(x). Suppose that g is strictly decreasing so that f'(f(x)) f'(x) < 0. One of the factors is positive and the other is negative. Since, by Darboux's theorem, f'(x) has the intermediate value property, there exists a q between x and f(x) such that f'(q) = 0. Then g'(q) = f'(f(q)) f'(q) = 0, a contradiction. Therefore g is not strictly decreasing.

    I've investigated the fixed points of f and found lots of interesting facts, but none of them seems to lead anywhere on this problem.

    Any ideas or suggestions? Thanks a lot.
  2. jcsd
  3. Jul 24, 2008 #2

    Ben Niehoff

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    You have your proof already. "Strictly decreasing" and "monotone decreasing" mean two different things. If g is monotone decreasing but not strictly decreasing, then g must be constant.
  4. Jul 24, 2008 #3
    Why is that?

    If g is monotone decreasing and not strictly decreasing, than means nothing more than that there's at least one u at which g'(u) = 0, i.e. g has at least one inflection point. However, if g were constant, it'd have g'(x) = 0 everywhere. Is there some standard result about monotone but not strictly decreasing functions that I'm unaware of?
    Last edited: Jul 24, 2008
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