For a 3-digit number in base 10, this number has a relatively simple divisibility test.
2.
Given the divisibility test for 99 is not a harshad number, since 9 + 9 = 18, and 99 is not divisible by 18.
3.
Digit sums and digital roots can be used for quick divisibility tests : a natural number is divisible by 3 or 9 if and only if its digit sum ( or digital root ) is divisible by 3 or 9, respectively.
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