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If n is greater than 15, then there is at least one number between n and 2n which is the product of three different primes. [Sierpinski] The number of trees with 15 vertices is prime. 215 - 51 is prime. The smallest multi-digit integer I such that 4*I+1 and 4*I-1 are both primes. [Russo] There are exactly 15 palindromic primes of length three. [Patterson] 15 is the smallest number which is product of two distinct odd primes. [Capelle] !15 - 1 is prime. Note that !15 represents subfactorial 15. [Gupta]
15 is the only number m such that m = 15 is the smallest emirpime. [Post] 15 is the (1+5)th Lucky Number. [Post] (F015 + F115 + F215 + F315 + F415) and (F015 + F115 + F215 + F315 + F415 + 6) are sexy primes. Note that the first five Fermat numbers are all prime. [Wesolowski] 15!-14!+ ... +3!-2!+1! is prime. [Silva]
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