Let $\sqrt 7=\sum_{i=0}^\infty a_i 7^{-i}, 0\le a_i \le 6$ be the expansion of $\sqrt 7$ in the base $7$.
I am curious about the following question: Is there a $K\in \mathbb{N}$ such that for any $n\ge K,$$n\ge K$, there exists an $ i\in [n, 2n] $$ i\in (n, 2n] $ for which $ a_i\ne 6.$$ a_i\ne 6$?