Given that P is a real polynomial of degree at most 2012 such that P(n)=2n for n=1,2,…,2012, what choice(s) of P(0) produce the minimal possible value of P(0)2+P(2013)2 ?
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Solution
Define \Delta^{1}(n)=P(n+1)-P(n)andΔi(n)=Δi−1(n+1)−Δi−1(n) for i>1. Since P(n) has degree at most 2012, we know that \Delta^{2012}(n)isconstant.Computing,weobtainΔ1(0)=2−P(0) and \Delta^{i}(0)=2^{i-1}for1<i \leq 2012.WeseethatcontinuingongivesΔ2012(0)=Δ2012(1)=P(0) and \Delta^{i}(2012-i)=2^{2013-i}for1 \leq i \leq 2011.Then,P(2013)=P(2012)+\Delta^{1}(2012)=\ldots=P(2012)+\Delta^{1}(2011)+\ldots+\Delta^{2012}(0)=P(0)+2^{2013}-2.Now,wewanttominimizethevalueofP(0)^{2}+P(2013)^{2}=2 P(0)^{2}+2 P(0)\left(2^{2013}-2\right)+\left(2^{2013}-2\right)^{2},butthisoccurssimplywhenP(0)=-\frac{1}{2}\left(2^{2013}-2\right)=1-2^{2012}$.
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