81393721 has 2 divisors, whose sum is σ = 81393722. Its totient is φ = 81393720.

The previous prime is 81393713. The next prime is 81393727. The reversal of 81393721 is 12739318.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 60279696 + 21114025 = 7764^2 + 4595^2 .

It is a cyclic number.

It is not a de Polignac number, because 81393721 - 2³ = 81393713 is a prime.

It is not a weakly prime, because it can be changed into another prime (81393727) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40696860 + 40696861.

It is an arithmetic number, because the mean of its divisors is an integer number (40696861).

Almost surely, 2^81393721 is an apocalyptic number.

It is an amenable number.

81393721 is a deficient number, since it is larger than the sum of its proper divisors (1).

81393721 is an equidigital number, since it uses as much as digits as its factorization.

81393721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 9072, while the sum is 34.

The square root of 81393721 is about 9021.8468730078. The cubic root of 81393721 is about 433.3747801908.

The spelling of 81393721 in words is "eighty-one million, three hundred ninety-three thousand, seven hundred twenty-one".