Question 17
(GBR) A sequence of integers $a_1,a_2,a_3,\ldots$ is defined as follows: $a_1=1$, and for $n\ge1$, $a_{n+1}$ is the smallest integer greater than $a_n$ such that $a_i+a_j\ne3a_k$ for any $i,j,k$ in $\{1,2,\ldots,n+1\}$, not necessarily distinct. Determine $a_{1998}$.