640621666 has 4 divisors (see below), whose sum is σ = 960932502. Its totient is φ = 320310832.

The previous prime is 640621649. The next prime is 640621669. The reversal of 640621666 is 666126046.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in only one way, i.e., 561927025 + 78694641 = 23705^2 + 8871^2 .

It is a super-2 number, since 2×640621666² = 820792237897231112, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (640621669) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 160155415 + ... + 160155418.

Almost surely, 2^640621666 is an apocalyptic number.

640621666 is a deficient number, since it is larger than the sum of its proper divisors (320310836).

640621666 is a wasteful number, since it uses less digits than its factorization.

640621666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 320310835.

The product of its (nonzero) digits is 62208, while the sum is 37.

The square root of 640621666 is about 25310.5050522505. The cubic root of 640621666 is about 862.0528146321.

The spelling of 640621666 in words is "six hundred forty million, six hundred twenty-one thousand, six hundred sixty-six".