81357721 has 4 divisors (see below), whose sum is σ = 81441972. Its totient is φ = 81273472.

The previous prime is 81357623. The next prime is 81357781. The reversal of 81357721 is 12775318.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 62094400 + 19263321 = 7880^2 + 4389^2 .

It is a cyclic number.

It is not a de Polignac number, because 81357721 - 2⁷ = 81357593 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81357781) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 40660 + ... + 42613.

It is an arithmetic number, because the mean of its divisors is an integer number (20360493).

Almost surely, 2^81357721 is an apocalyptic number.

It is an amenable number.

81357721 is a deficient number, since it is larger than the sum of its proper divisors (84251).

81357721 is an equidigital number, since it uses as much as digits as its factorization.

81357721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 84250.

The product of its digits is 11760, while the sum is 34.

The square root of 81357721 is about 9019.8514954516. The cubic root of 81357721 is about 433.3108776661.

The spelling of 81357721 in words is "eighty-one million, three hundred fifty-seven thousand, seven hundred twenty-one".