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Algebra Difficulty 6.2 National olympiad Prove it Brazil

Given positive integers nn, a1a_1, a2a_2, \dots, ana_n, define q0=1q_0 = 1, q1=a1q_1 = a_1 and qk+1=ak+1qk+qk1q_{k+1} = a_{k+1}q_k + q_{k-1}, for 1kn11 \le k \le n-1.
Prove that, given c>1c > 1, there exists K>0K > 0 such that, for all M>KM > K, there exist a positive integer nn and a1,a2,,an{1,2}a_1, a_2, \dots, a_n \in \{1, 2\} such that Mqn<cMM \le q_n < c \cdot M.

Solution

We will choose two large positive integers rr, ss, and take m=r+sm = r + s, aj=1a_j = 1 for 1jr1 \le j \le r and aj=2a_j = 2 for r+1jr+s=mr + 1 \le j \le r + s = m.
We have qk+1=qk+qk1q_{k+1} = q_k + q_{k-1}, for 1kr11 \le k \le r-1, and so qj=Fj+1q_j = F_{j+1}, for 0jr0 \le j \le r, where
Fj=15((1+52)j(152)j)=1+o(1)5(1+52)j F_j = \frac{1}{\sqrt{5}} \left( \left( \frac{1+\sqrt{5}}{2} \right)^j - \left( \frac{1-\sqrt{5}}{2} \right)^j \right) = \frac{1+o(1)}{\sqrt{5}} \left( \frac{1+\sqrt{5}}{2} \right)^j
is the jj-th term of Fibonacci's sequence, for j1j \ge 1. So qj=1+o(1)5(1+52)j+1q_j = \frac{1+o(1)}{\sqrt{5}} (\frac{1+\sqrt{5}}{2})^{j+1} for large jj.

On the other hand, we have qk+1=2qk+qk1q_{k+1} = 2q_k + q_{k-1} for rkm1r \le k \le m-1, and so qr+j=uj+1qr+ujqr1q_{r+j} = u_{j+1}q_r + u_j q_{r-1}, where (uj)j0(u_j)_{j \ge 0} is the sequence given by u0=0u_0 = 0, u1=1u_1 = 1 and uk+2=2uk+1+uku_{k+2} = 2u_{k+1} + u_k, for k0k \ge 0. Since
uk=122((1+2)k(12)k)=1+o(1)22(1+2)k,for k0, u_k = \frac{1}{2\sqrt{2}}((1+\sqrt{2})^k - (1-\sqrt{2})^k) = \frac{1+o(1)}{2\sqrt{2}}(1+\sqrt{2})^k, \quad \text{for } k \ge 0,
we get
qr+j=1+o(1)22((1+2)qr+qr1)(1+2)j=1+o(1)210(1+2+512)(1+2)j(1+52)r+1=(1+o(1))4+10+285(1+2)j(1+52)r+1, \begin{align*} q_{r+j} &= \frac{1+o(1)}{2\sqrt{2}}((1+\sqrt{2})q_r + q_{r-1})(1+\sqrt{2})^j \\ &= \frac{1+o(1)}{2\sqrt{10}}\left(1+\sqrt{2}+\frac{\sqrt{5}-1}{2}\right)(1+\sqrt{2})^j\left(\frac{1+\sqrt{5}}{2}\right)^{r+1} \\ &= (1+o(1))\frac{4+\sqrt{10}+\sqrt{2}}{8\sqrt{5}}(1+\sqrt{2})^j\left(\frac{1+\sqrt{5}}{2}\right)^{r+1}, \end{align*}
provided that jj and rr are large.

Since log(1+2)/log(1+52)\log(1 + \sqrt{2}) / \log(\frac{1+\sqrt{5}}{2}) is irrational, the result follows (by taking logarithms) from the elementary fact below:
Given α\alpha, β>0\beta > 0 such that α/β\alpha/\beta is irrational, ϵ>0\epsilon > 0 and r>0r > 0, there is x0>0x_0 > 0 such that, for every xRx \in \mathbb{R}, xx0x \ge x_0, there are positive integers mm, nrn \ge r such that mα+nβx<ϵ|m\alpha + n\beta - x| < \epsilon.

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