Given positive integers n, a1, a2, …, an, define q0=1, q1=a1 and qk+1=ak+1qk+qk−1, for 1≤k≤n−1. Prove that, given c>1, there exists K>0 such that, for all M>K, there exist a positive integer n and a1,a2,…,an∈{1,2} such that M≤qn<c⋅M.
Solution
We will choose two large positive integers r, s, and take m=r+s, aj=1 for 1≤j≤r and aj=2 for r+1≤j≤r+s=m. We have qk+1=qk+qk−1, for 1≤k≤r−1, and so qj=Fj+1, for 0≤j≤r, where Fj=51(21+5)j−(21−5)j=51+o(1)(21+5)j is the j-th term of Fibonacci's sequence, for j≥1. So qj=51+o(1)(21+5)j+1 for large j.
On the other hand, we have qk+1=2qk+qk−1 for r≤k≤m−1, and so qr+j=uj+1qr+ujqr−1, where (uj)j≥0 is the sequence given by u0=0, u1=1 and uk+2=2uk+1+uk, for k≥0. Since uk=221((1+2)k−(1−2)k)=221+o(1)(1+2)k,for k≥0, we get qr+j=221+o(1)((1+2)qr+qr−1)(1+2)j=2101+o(1)(1+2+25−1)(1+2)j(21+5)r+1=(1+o(1))854+10+2(1+2)j(21+5)r+1, provided that j and r are large.
Since log(1+2)/log(21+5) is irrational, the result follows (by taking logarithms) from the elementary fact below: Given α, β>0 such that α/β is irrational, ϵ>0 and r>0, there is x0>0 such that, for every x∈R, x≥x0, there are positive integers m, n≥r such that ∣mα+nβ−x∣<ϵ.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: MathNet,
licensed CC-BY-4.0.
Statement and solution reproduced as published; topic and difficulty added by this site.