42

This number is a composite.

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42 is the smallest number k that is equal to the sum of the nonprime proper divisors of k, i.e., 42 = 1 + 6 + 14 + 21. Integers with this property are the product of a perfect number and a Carol prime. [Honaker]

42 divides the sum of digits of the first 42 prime numbers, i.e., 42 divides (2 + 3 + 5 + 7 + 1+1 + … + 1+7+9 + 1+8+1) or 378. Can you find the first prime with this property? See PC71. [Honaker]