6661

6661 is a prime number.

6659 and 6661 form a twin prime pair.

6661 is the smallest beastly (containing the string 666) prime number.

6661 and its inversion, 1999, are both primes.

6661 uses only the digits 0, 1, and 5 in bases 6 (50501) and 8 (15005).

6661 has a representation as a sum of two squares: 6661 = 10² + 81².

6661 is the hypotenuse of a primitive Pythagorean triple: 6661² = 1620² + 6461².

Source: Number Gossip