646686666 has 48 divisors (see below), whose sum is σ = 1492362144. Its totient is φ = 209730384.

The previous prime is 646686661. The next prime is 646686731. The reversal of 646686666 is 666686646.

646686666 is a nontrivial binomial coefficient, being equal to C(35964, 2).

It is a Harshad number since it is a multiple of its sum of digits (54).

It is not an unprimeable number, because it can be changed into a prime (646686661) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 1 + ... + 35963.

It is an arithmetic number, because the mean of its divisors is an integer number (31090878).

Almost surely, 2^646686666 is an apocalyptic number.

646686666 is the 35963-rd triangular number and also the 17982-nd hexagonal number.

It is a practical number, because each smaller number is the sum of distinct divisors of 646686666, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (746181072).

646686666 is an abundant number, since it is smaller than the sum of its proper divisors (845675478).

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

646686666 is a wasteful number, since it uses less digits than its factorization.

646686666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 36017 (or 36005 counting only the distinct ones).

The product of its digits is 8957952, while the sum is 54.

The square root of 646686666 is about 25430.0347227447. The cubic root of 646686666 is about 864.7647309191.

The spelling of 646686666 in words is "six hundred forty-six million, six hundred eighty-six thousand, six hundred sixty-six".