21661508664 has 96 divisors (see below), whose sum is σ = 64121466240. Its totient is φ = 6576537600.

The previous prime is 21661508653. The next prime is 21661508677. The reversal of 21661508664 is 46680516612.

21661508664 is a `hidden beast` number, since 21 + 6 + 61 + 508 + 6 + 64 = 666.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 247497 + ... + 323384.

It is an arithmetic number, because the mean of its divisors is an integer number (667931940).

Almost surely, 2^21661508664 is an apocalyptic number.

21661508664 is a gapful number since it is divisible by the number (24) formed by its first and last digit.

It is an amenable number.

21661508664 is an abundant number, since it is smaller than the sum of its proper divisors (42459957576).

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

21661508664 is a wasteful number, since it uses less digits than its factorization.

21661508664 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 570941 (or 570934 counting only the distinct ones).

The product of its (nonzero) digits is 414720, while the sum is 45.

The spelling of 21661508664 in words is "twenty-one billion, six hundred sixty-one million, five hundred eight thousand, six hundred sixty-four".