640621661 has 4 divisors (see below), whose sum is σ = 640722300. Its totient is φ = 640521024.

The previous prime is 640621649. The next prime is 640621669. The reversal of 640621661 is 166126046.

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

It can be written as a sum of positive squares in 2 ways, for example, as 512796025 + 127825636 = 22645^2 + 11306^2 .

It is a cyclic number.

It is not a de Polignac number, because 640621661 - 2¹⁰ = 640620637 is a prime.

It is a super-2 number, since 2×640621661² = 820792225084797842, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

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 in 3 ways as a sum of consecutive naturals, for example, 40076 + ... + 53733.

It is an arithmetic number, because the mean of its divisors is an integer number (160180575).

Almost surely, 2^640621661 is an apocalyptic number.

It is an amenable number.

640621661 is a deficient number, since it is larger than the sum of its proper divisors (100639).

640621661 is an equidigital number, since it uses as much as digits as its factorization.

640621661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 100638.

The product of its (nonzero) digits is 10368, while the sum is 32.

The square root of 640621661 is about 25310.5049534773. The cubic root of 640621661 is about 862.0528123894.

The spelling of 640621661 in words is "six hundred forty million, six hundred twenty-one thousand, six hundred sixty-one".