90

This number is a composite.

There are only 8 positive integers n for which the number of primes ≤ n (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]