21720939891 has 32 divisors (see below), whose sum is σ = 34168189440. Its totient is φ = 12015972864.

The previous prime is 21720939887. The next prime is 21720939917. The reversal of 21720939891 is 19893902712.

It is not a de Polignac number, because 21720939891 - 2² = 21720939887 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21720939811) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 4396 + ... + 208473.

It is an arithmetic number, because the mean of its divisors is an integer number (1067755920).

Almost surely, 2^21720939891 is an apocalyptic number.

21720939891 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

21720939891 is a deficient number, since it is larger than the sum of its proper divisors (12447249549).

21720939891 is a wasteful number, since it uses less digits than its factorization.

21720939891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 213035.

The product of its (nonzero) digits is 489888, while the sum is 51.

The spelling of 21720939891 in words is "twenty-one billion, seven hundred twenty million, nine hundred thirty-nine thousand, eight hundred ninety-one".