75

This number is a composite.

+ 275 + 75 is prime. [Luhn]

+ 75 is the smallest non-prime deficient number whose sum as well as product of aliquot divisors is a perfect square. [Gupta]

+ The first integer n such that the absolute value of the form n-2^k creates eight primes for eight successive values of k. Can you find another number with the same property? [Loungrides]

(There is one curio for this number that has not yet been approved by an editor.)

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