604
This number is a composite.
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604 is the only three-digit number whose sum of its single and double factorials plus one is prime. I.e., 604! + 604!2 + 1 is prime. [Opao]
Nebula NGC 604 was discovered by William Herschel in 1784. It is 3 million light years away. Note that 604*1784+3 is prime. [Opao]
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