Question 65
Let $\{a_n\}$ be a sequence of positive integers. It is given that $a_1 = 1$, and for $n \ge 1$, $a_{n+1}$ is the smallest positive integer greater than $a_n$ which satisfies the following condition: for any integers $i, j, k$, with $1 \le i, j, k \le n+1$, $a_i + a_j \ne 3a_k$. Find $a_{2015}$.