666

666 = 2 x 3 x 3 x 37.

666 is the 36th triangular number; it is the sum of the first 36 positive integers. It is the largest triangular number that consists of the same repeated digit.

666 is the sum of the squares of two consecutive triangular numbers: 666 = (15 x 15) + (21 x 21).

666 is the sum of two consecutive palindromic primes: 666 = 313 + 353.

666 is the sum of the squares of the first seven prime numbers: 666 = 2² + 3² + 5² + 7² + 11² + 13² + 17².

666 has a representation as a sum of two squares: 666 = 15² + 21².

There are exactly 666 twin primes less than 6⁶ + 666.

666 has the representation 3⁶ - 63.

666 lists in their proper order the letters used for numbers in the Roman numeration system: D (500) + C (100) + L (50) + X (10) + V (5) + I (1), or DCLXVI.

666 is the sum of all the numbers on a typical roulette wheel.

Source: Prime Curios!