ISL 1998 N8

Let $a_{0},a_{1},a_{2},\ldots$ be an increasing sequence of nonnegative integers such that every nonnegative integer can be expressed uniquely in the form $a_{i}+2a_{j}+4a_{k}$ , where $i,j$ and $k$ are not necessarily distinct. Determine $a_{1998}$ .

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