81357921 has 12 divisors (see below), whose sum is σ = 118198548. Its totient is φ = 53924064.

The previous prime is 81357907. The next prime is 81357929. The reversal of 81357921 is 12975318.

81357921 is a `hidden beast` number, since 81 + 3 + 579 + 2 + 1 = 666.

It can be written as a sum of positive squares in 2 ways, for example, as 52359696 + 28998225 = 7236^2 + 5385^2 .

It is not a de Polignac number, because 81357921 - 2¹⁰ = 81356897 is a prime.

It is a super-2 number, since 2×81357921² = 13238222618884482, which contains 22 as substring.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (81357929) 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, 24570 + ... + 27683.

It is an arithmetic number, because the mean of its divisors is an integer number (9849879).

Almost surely, 2^81357921 is an apocalyptic number.

It is an amenable number.

81357921 is a deficient number, since it is larger than the sum of its proper divisors (36840627).

81357921 is a wasteful number, since it uses less digits than its factorization.

81357921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 52432 (or 52429 counting only the distinct ones).

The product of its digits is 15120, while the sum is 36.

The square root of 81357921 is about 9019.8625821018. The cubic root of 81357921 is about 433.3112327322.

The spelling of 81357921 in words is "eighty-one million, three hundred fifty-seven thousand, nine hundred twenty-one".