• 666 can be written using four 4's:

See also 113.

---

666 has 12 divisors (see below), whose sum is σ = 1482. Its totient is φ = 216.

The previous prime is 661. The next prime is 673.

666 is nontrivially palindromic in base 4, base 10 and base 13.

666 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

666 is an esthetic number in base 8, because in such base its adjacent digits differ by 1.

666 is a nontrivial binomial coefficient, being equal to C(37, 2).

It can be written as a sum of positive squares in only one way, i.e., 441 + 225 = 21^2 + 15^2 .

It is a Smith number, since the sum of its digits (18) coincides with the sum of the digits of its prime factors.

It is a Harshad number since it is a multiple of its sum of digits (18), and also a Moran number because the ratio is a prime number: 37 = 666 / (6 + 6 + 6).

It is a nude number because it is divisible by every one of its digits.

666 is an undulating number in base 6 and base 13.

666 is a modest number, since divided by 66 gives 6 as remainder.

666 is a nontrivial repdigit in base 10.

It is a plaindrome in base 10, base 11, base 14 and base 16.

It is a nialpdrome in base 9 and base 10.

It is a zygodrome in base 10.

It is not an unprimeable number, because it can be changed into a prime (661) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1 + ... + 36.

2⁶⁶⁶ is an apocalyptic number.

666 is the 36-th triangular number.

It is a practical number, because each smaller number is the sum of distinct divisors of 666, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (741).

666 is an abundant number, since it is smaller than the sum of its proper divisors (816).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

666 is a wasteful number, since it uses less digits than its factorization.

666 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 45 (or 42 counting only the distinct ones).

The product of its digits is 216, while the sum is 18.

The square root of 666 is about 25.8069758011. The cubic root of 666 is about 8.7328917413.

The spelling of 666 in words is "six hundred sixty-six", and thus it is an aban number and an oban number.