81949981921 has 2 divisors, whose sum is σ = 81949981922. Its totient is φ = 81949981920.

The previous prime is 81949981913. The next prime is 81949981933. The reversal of 81949981921 is 12918994918.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 76942438225 + 5007543696 = 277385^2 + 70764^2 .

It is a cyclic number.

It is not a de Polignac number, because 81949981921 - 2³ = 81949981913 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 81949981921.

It is not a weakly prime, because it can be changed into another prime (81949981991) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40974990960 + 40974990961.

It is an arithmetic number, because the mean of its divisors is an integer number (40974990961).

Almost surely, 2^81949981921 is an apocalyptic number.

It is an amenable number.

81949981921 is a deficient number, since it is larger than the sum of its proper divisors (1).

81949981921 is an equidigital number, since it uses as much as digits as its factorization.

81949981921 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3359232, while the sum is 61.

The spelling of 81949981921 in words is "eighty-one billion, nine hundred forty-nine million, nine hundred eighty-one thousand, nine hundred twenty-one".