81857879921 has 16 divisors (see below), whose sum is σ = 84884171088. Its totient is φ = 78875596800.

The previous prime is 81857879849. The next prime is 81857879981. The reversal of 81857879921 is 12997875818.

It can be written as a sum of positive squares in 8 ways, for example, as 1284218896 + 80573661025 = 35836^2 + 283855^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81857879921 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81857879981) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 5515691 + ... + 5530511.

It is an arithmetic number, because the mean of its divisors is an integer number (5305260693).

Almost surely, 2^81857879921 is an apocalyptic number.

It is an amenable number.

81857879921 is a deficient number, since it is larger than the sum of its proper divisors (3026291167).

81857879921 is a wasteful number, since it uses less digits than its factorization.

81857879921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 16292.

The product of its digits is 20321280, while the sum is 65.

The spelling of 81857879921 in words is "eighty-one billion, eight hundred fifty-seven million, eight hundred seventy-nine thousand, nine hundred twenty-one".