81431721 has 12 divisors (see below), whose sum is σ = 134427072. Its totient is φ = 46532376.

The previous prime is 81431689. The next prime is 81431737. The reversal of 81431721 is 12713418.

It is not a de Polignac number, because 81431721 - 2⁵ = 81431689 is a prime.

It is a super-2 number, since 2×81431721² = 13262250370043682, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81430721) 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 in 11 ways as a sum of consecutive naturals, for example, 646221 + ... + 646346.

It is an arithmetic number, because the mean of its divisors is an integer number (11202256).

Almost surely, 2^81431721 is an apocalyptic number.

It is an amenable number.

81431721 is a deficient number, since it is larger than the sum of its proper divisors (52995351).

81431721 is a wasteful number, since it uses less digits than its factorization.

81431721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1292580 (or 1292577 counting only the distinct ones).

The product of its digits is 1344, while the sum is 27.

The square root of 81431721 is about 9023.9526262054. The cubic root of 81431721 is about 433.4422124173.

The spelling of 81431721 in words is "eighty-one million, four hundred thirty-one thousand, seven hundred twenty-one".