90

This number is a composite.

+ There are only 8 positive integers n for which the number of primesn (pi(n)) equals the number of positive integers ≤ n relative prime to n (phi(n)). They are 2, 3, 4, 8, 10, 14, 20, and 90. [Moser]

+ (903 - 1)/(90 - 1) is a Mersenne prime. [Goormaghtigh]

+ The smallest number n such that it can be represented as sum of each of the terms of a set of six consecutive primes, i.e., {17, 19, 23, 29, 31, 37}, with a term of another set of six consecutive primes, {73, 71, 67, 61, 59, 53}. [Loungrides]

(There are 3 curios for this number that have not yet been approved by an editor.)

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