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Analysis Question-differentiabillity, continuity

  1. Jul 16, 2012 #1
    Analysis Question--differentiabillity, continuity

    1. The problem statement, all variables and given/known data
    Suppose [itex]f:\mathbb{R}\to\mathbb{R}[/itex] is a [itex]C^\infty[/itex] function which satisfies the equation [itex]f''(x)=-x^2f(x)[/itex] along with [itex]f(0)=1[/itex], [itex]f'(0)=0[/itex]. Prove that there is an [itex]a>0[/itex] such that [itex]f(a)=0[/itex]. Do not use any results from differential equations. Thank you.

    2. Relevant equations

    3. The attempt at a solution
    Since f is continuously differentiable there is a [itex](0,\delta)[/itex] interval in which f is concave down. If we can show f is also decreasing then it follows that f must cross the x axis before changing concavity and increasing because there can be no cusps as f is differentiable everywhere. I have no idea if that is the right track. Thank you.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: Jul 16, 2012
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  3. Jul 16, 2012 #2


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  4. Jul 18, 2012 #3
    Re: Analysis Question--differentiabillity, continuity

    Not true. A counterexample sin(x)+2 goes between convex and concave without crossing x.

    Perhaps it'd be easier to prove by contradiction.
    f(0)=1>0, by continuity, in the immediate neighborhood xE[0,c], f(x)>0. Let's (erroneously) assume f(x)>0 for all x>0. since f''(x)<0, f(x) concave down, it follows that
    f(x)<f(c)+f'(c)(x-c) for all x>0 and c>0
    You can prove f'(x)<0 for all x>0 (if the assumption is true), hence f'(x)=-|f'(x)|.

    Now, let x=ζ=c+f(c)/|f'(c)|, it leads to
    f(ζ)<0 where ζ>c>0. Now this contradicts with the assumption that f(x)>0 for all x>0.

    Not pretty, just to bounce some idea.
    Last edited: Jul 18, 2012
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