# 27

This number is a composite. The smallest 2-digit number in which the sum of digits is equal to the sum of prime factors (27 = 3 * 3 * 3 and 3 + 3 + 3 = 9). [Russo] Vaughan (1977) showed that all integers are the sum of at most 27 prime numbers. 2 and 7 are the smallest and largest single-digit primes. 2 + 3 + 4 + 5 + 6 + 7 = 27. [Stone] The product of the first 27 Mersenne prime exponents (minus one) is a 27 reversed-digit prime. Smaller of the smallest pair of consecutive numbers with 3 prime factors. [Gupta] The smallest number that is both a Smith number and a SPRP for a base less than one hundred. [H. Brown] The smallest composite number n > 1 such that n! + 1 is prime. [Capelle] π(2!!) * π(7!!) = 27. [Firoozbakht] 27^27 + (2*7)^(2*7) is prime. [Silva] π(27) = 2 + 7. [Kumar] The smallest composite number which cannot be expressed as the sum of two primes. [Wesolowski] The only prime cube that is one less than an even perfect number. [Gudipati] The sum of all composite digits (4+6+8+9) equals the first odd prime cubed. [Silva]

(There are 8 curios for this number that have not yet been approved by an editor.)