What Numbers Can Be Harshad Numbers?
Given the divisibility test for 9, one might be tempted to generalize that all numbers divisible by 9 are also Harshad numbers. But for the purpose of determining the Harshadness of n, the digits of n can only be added up once and n must be divisible by that sum; otherwise, it is not a Harshad number. For example, 99 is not a Harshad number, since 9 + 9 = 18, and 99 is not divisible by 18.
The base number (and furthermore, its powers) will always be a Harshad number in its own base, since it will be represented as "10" and 1 + 0 = 1.
For a prime number to also be a Harshad number it must be less than or equal to the base number. Otherwise, the digits of the prime will add up to a number that is more than 1 but less than the prime and, obviously, it will not be divisible. For example: 11 is not Harshad in base 10 because the sum of its digits "11" is 1+1=2 and 11 is not divisible by 2, while in in hexadecimal the number 11 may be represented as "B", the sum of whose digits is also B and clearly B is divisible by B, ergo it is Harshad in base 16.
Although the sequence of factorials starts with Harshad numbers in base 10, not all factorials are Harshad numbers. 432! is the first that is not.
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