How to show that this decision problem is in co-NP?

Question

Given a set of strictly positive numbers $a_1, ..., a_n$, the problem is to determine if $\lfloor n/2 \rfloor$ different indexes $i_1, ..., i_{\lfloor n/2 \rfloor}$ exist so that $$\frac{a_{i_j}}{a_{i_{j-1}}} = \frac{a_{i_{j+1}}}{a_{i_j}}$$ for $2 \leq j \leq \lfloor n/2 \rfloor$.

How to show that this problem is in co-NP ?