AlgebraDifficulty 8.1Prove itTeam selection tests for IMO 2018 · Saudi Arabia · 2018
f(x)=(x−F1)(x−F2)…(x−F3030) with (Fn) is the Fibonacci sequence, which defined as F1=1,F2=2,Fn+2=Fn+1+Fn,n≥1. Suppose that on the range (F1,F3030), the function ∣f(x)∣ takes on the maximum value at x=x0. Prove that x0>22018.
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Official solution
We will prove that x0∈(F3029,F3030) by showing that for all x∗∈(F1,F3029], there is some x∗∗∈(F3029,F3030) for which ∣f(x∗∗)∣>∣f(x∗)∣.
Indeed, if x∗∈{F1,F2,…,F3029} then ∣f(x∗)∣=0 which is obvious.
Suppose that x∗∈(Fk,Fk+1) for 1≤k≤3028 then put m=x∗−Fk, we choose x∗∗=F3030−m. We need ∣f(x∗∗)∣>∣f(x∗)∣⇔i=1∏3030(x∗∗−Fi)>i=1∏3030(x∗−Fi). Each side has 3030 positive factors then we will make pair one of the left and one of the right such that the value of the right is bigger (except the case i=k then ∣x∗−Fk∣=m=∣x∗∗−F3030∣ ). For details:
1. If i=1,2,…,k, it is clearly that x∗∗−Fi>x∗−Fi>0.
2. If 1≤i≤3030−k, we have ∣x∗−Fk+i∣<∣x∗∗−F3030−i∣. Note that we just consider the separated ranges, i.e. k+i≤3030−i. Otherwise, some ranges are overlap then we remove that part, then Fk+i−x∗=(Fk+1−x∗)+(Fk+2−Fk+1)+⋯+(Fk+i−Fk+i−1)<(x∗∗−F3029)+(F3029−F3028)+⋯+(F3030−(i−1)−F3030−i)=x∗∗−F3030−i Hence, the statement is proved. From here, we have x0>F3029=51(21+5)3030−(21−5)3030>51(21+5)3029>(21+5)3027. It is easy to check that 21+5>232 then substitute into the above inequality, we get x0>22018.
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