21721660 has 24 divisors (see below), whose sum is σ = 47599776. Its totient is φ = 8310720.

The previous prime is 21721621. The next prime is 21721717. The reversal of 21721660 is 6612712.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 23151 + ... + 24070.

It is an arithmetic number, because the mean of its divisors is an integer number (1983324).

Almost surely, 2^21721660 is an apocalyptic number.

21721660 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21721660 is an abundant number, since it is smaller than the sum of its proper divisors (25878116).

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

21721660 is a wasteful number, since it uses less digits than its factorization.

21721660 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 47253 (or 47251 counting only the distinct ones).

The product of its (nonzero) digits is 1008, while the sum is 25.

The square root of 21721660 is about 4660.6501692360. The cubic root of 21721660 is about 279.0172177981.

The spelling of 21721660 in words is "twenty-one million, seven hundred twenty-one thousand, six hundred sixty".