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Is the sum of digits of $3^{1000}$ divisible by $7$?

Is the sum of digits of $3^{1000}$ a multiple of $7$? The sum of the digits of $3^{1000}$ can be computed using a computer. It is equal to $2142$, so the answer is positive. … The results below are known: $3^{1000}$ has $478$ digits, and so the sum is at most $4302$ ($9\cdot478$). This sum is a multiple of $9$. The last four digits of $3^{1000}$ are $0001$. …
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