81320021 has 2 divisors, whose sum is σ = 81320022. Its totient is φ = 81320020.

The previous prime is 81320017. The next prime is 81320023. The reversal of 81320021 is 12002318.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 42706225 + 38613796 = 6535^2 + 6214^2 .

It is a cyclic number.

It is not a de Polignac number, because 81320021 - 2² = 81320017 is a prime.

It is a super-2 number, since 2×81320021² = 13225891630880882, which contains 22 as substring.

Together with 81320023, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81320023) 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 as a sum of consecutive naturals, namely, 40660010 + 40660011.

It is an arithmetic number, because the mean of its divisors is an integer number (40660011).

Almost surely, 2^81320021 is an apocalyptic number.

It is an amenable number.

81320021 is a deficient number, since it is larger than the sum of its proper divisors (1).

81320021 is an equidigital number, since it uses as much as digits as its factorization.

81320021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 96, while the sum is 17.

The square root of 81320021 is about 9017.7614184453. The cubic root of 81320021 is about 433.2439373118.

Adding to 81320021 its reverse (12002318), we get a palindrome (93322339).

The spelling of 81320021 in words is "eighty-one million, three hundred twenty thousand, twenty-one".