81372721 has 2 divisors, whose sum is σ = 81372722. Its totient is φ = 81372720.

The previous prime is 81372691. The next prime is 81372749. The reversal of 81372721 is 12727318.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 81360400 + 12321 = 9020^2 + 111^2 .

It is a cyclic number.

It is not a de Polignac number, because 81372721 - 2⁹ = 81372209 is a prime.

It is a super-3 number, since 3×81372721³ (a number of 25 digits) contains 333 as substring.

It is not a weakly prime, because it can be changed into another prime (81372821) by changing a digit.

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 as a sum of consecutive naturals, namely, 40686360 + 40686361.

It is an arithmetic number, because the mean of its divisors is an integer number (40686361).

Almost surely, 2^81372721 is an apocalyptic number.

It is an amenable number.

81372721 is a deficient number, since it is larger than the sum of its proper divisors (1).

81372721 is an equidigital number, since it uses as much as digits as its factorization.

81372721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 4704, while the sum is 31.

The square root of 81372721 is about 9020.6829564063. The cubic root of 81372721 is about 433.3375060085.

The spelling of 81372721 in words is "eighty-one million, three hundred seventy-two thousand, seven hundred twenty-one".