81321721 has 16 divisors (see below), whose sum is σ = 91889280. Its totient is φ = 71406720.

The previous prime is 81321697. The next prime is 81321743. The reversal of 81321721 is 12712318.

It is a happy number.

It is not a de Polignac number, because 81321721 - 2⁵ = 81321689 is a prime.

It is a super-2 number, since 2×81321721² = 13226444612803682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81321791) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 62445 + ... + 63733.

It is an arithmetic number, because the mean of its divisors is an integer number (5743080).

Almost surely, 2^81321721 is an apocalyptic number.

It is an amenable number.

81321721 is a deficient number, since it is larger than the sum of its proper divisors (10567559).

81321721 is a wasteful number, since it uses less digits than its factorization.

81321721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1536.

The product of its digits is 672, while the sum is 25.

The square root of 81321721 is about 9017.8556763789. The cubic root of 81321721 is about 433.2469562877.

The spelling of 81321721 in words is "eighty-one million, three hundred twenty-one thousand, seven hundred twenty-one".