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 ?
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Define Δ1(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 Δ2012(n)isconstant.Computing,weobtainΔ1(0)=2−P(0) and Δi(0)=2i−1for1<i ≤ 2012.WeseethatcontinuingongivesΔ2012(0)=Δ2012(1)=P(0) and Δi(2012−i)=22013−ifor1 ≤i≤ 2011.Then,P(2013)=P(2012)+Δ1(2012)=…=P(2012)+Δ1(2011)+…+Δ2012(0)=P(0)+22013−2.Now,wewanttominimizethevalueofP(0)2+P(2013)2=2P(0)2+2P(0)(22013−2)+(22013−2)2, but this occurs simply when P(0)=−21(22013−2)=1−22012$.
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