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Putnam 2019 A6

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December 10, 2019
PutnamPutnam 2019

Problem Statement

Let gg be a real-valued function that is continuous on the closed interval [0,1][0,1] and twice differentiable on the open interval (0,1)(0,1).  Suppose that for some real number r>1r>1, limx0+g(x)xr=0. \lim_{x\to 0^+}\frac{g(x)}{x^r} = 0. Prove that either limx0+g(x)=0orlim supx0+xrg(x)=. \lim_{x\to 0^+}g'(x) = 0\qquad\text{or}\qquad \limsup_{x\to 0^+}x^r|g''(x)|= \infty.