15721921 has 2 divisors, whose sum is σ = 15721922. Its totient is φ = 15721920.

The previous prime is 15721919. The next prime is 15721967. The reversal of 15721921 is 12912751.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15689521 + 32400 = 3961^2 + 180^2 .

It is a cyclic number.

It is not a de Polignac number, because 15721921 - 2¹ = 15721919 is a prime.

Together with 15721919, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 15721921.

It is not a weakly prime, because it can be changed into another prime (15721021) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 7860960 + 7860961.

It is an arithmetic number, because the mean of its divisors is an integer number (7860961).

Almost surely, 2^15721921 is an apocalyptic number.

It is an amenable number.

15721921 is a deficient number, since it is larger than the sum of its proper divisors (1).

15721921 is an equidigital number, since it uses as much as digits as its factorization.

15721921 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1260, while the sum is 28.

The square root of 15721921 is about 3965.0877669984. The cubic root of 15721921 is about 250.5158468759.

The spelling of 15721921 in words is "fifteen million, seven hundred twenty-one thousand, nine hundred twenty-one".