15721239721 has 12 divisors (see below), whose sum is σ = 17285331700. Its totient is φ = 14287933440.

The previous prime is 15721239697. The next prime is 15721239733. The reversal of 15721239721 is 12793212751.

It can be written as a sum of positive squares in 2 ways, for example, as 6769998400 + 8951241321 = 82280^2 + 94611^2 .

It is not a de Polignac number, because 15721239721 - 2¹⁵ = 15721206953 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (15721239791) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 453865 + ... + 487273.

Almost surely, 2^15721239721 is an apocalyptic number.

15721239721 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

15721239721 is a deficient number, since it is larger than the sum of its proper divisors (1564091979).

15721239721 is a wasteful number, since it uses less digits than its factorization.

15721239721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 37320 (or 37309 counting only the distinct ones).

The product of its digits is 52920, while the sum is 40.

The spelling of 15721239721 in words is "fifteen billion, seven hundred twenty-one million, two hundred thirty-nine thousand, seven hundred twenty-one".