It is divisible.
If we split up the number 𝑛 having 81 ones into 9 blocks of 9 ones each, like this:
𝑛=111111111|111111111|111111111|111111111|111111111|111111111|111111111|111111111|111111111=111111111|111111111|111111111|111111111|111111111|111111111|111111111|111111111|111111111
we notice that
𝑛=111111111×10000000100000001000000010000000100000001000000010000000100000001=111111111×10000000100000001000000010000000100000001000000010000000100000001.
Both of the above numbers have exactly 9 ones, and no other nonzero digits. Hence, they are both divisible by 9, because the sum of their digits is divisible by 9. Consequently, 𝑛 is divisible by 9×9=819×9=81, as required.
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