81137721 has 16 divisors (see below), whose sum is σ = 129014784. Its totient is φ = 44348304.

The previous prime is 81137713. The next prime is 81137741. The reversal of 81137721 is 12773118.

It is not a de Polignac number, because 81137721 - 2³ = 81137713 is a prime.

It is not an unprimeable number, because it can be changed into a prime (81137741) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 83511 + ... + 84476.

It is an arithmetic number, because the mean of its divisors is an integer number (8063424).

Almost surely, 2^81137721 is an apocalyptic number.

It is an amenable number.

81137721 is a deficient number, since it is larger than the sum of its proper divisors (47877063).

81137721 is a wasteful number, since it uses less digits than its factorization.

81137721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 168020.

The product of its digits is 2352, while the sum is 30.

The square root of 81137721 is about 9007.6479171868. The cubic root of 81137721 is about 432.9199520629.

The spelling of 81137721 in words is "eighty-one million, one hundred thirty-seven thousand, seven hundred twenty-one".