81904000721 has 2 divisors, whose sum is σ = 81904000722. Its totient is φ = 81904000720.

The previous prime is 81904000699. The next prime is 81904000727. The reversal of 81904000721 is 12700040918.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 74933587600 + 6970413121 = 273740^2 + 83489^2 .

It is a cyclic number.

It is not a de Polignac number, because 81904000721 - 2³⁰ = 80830258897 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (81904000727) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40952000360 + 40952000361.

It is an arithmetic number, because the mean of its divisors is an integer number (40952000361).

Almost surely, 2^81904000721 is an apocalyptic number.

It is an amenable number.

81904000721 is a deficient number, since it is larger than the sum of its proper divisors (1).

81904000721 is an equidigital number, since it uses as much as digits as its factorization.

81904000721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 4032, while the sum is 32.

The spelling of 81904000721 in words is "eighty-one billion, nine hundred four million, seven hundred twenty-one", and thus it is an aban number.