81920021 has 2 divisors, whose sum is σ = 81920022. Its totient is φ = 81920020.

The previous prime is 81919993. The next prime is 81920023. The reversal of 81920021 is 12002918.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 60621796 + 21298225 = 7786^2 + 4615^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81920021 is a prime.

It is a Sophie Germain prime.

Together with 81920023, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81920023) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40960010 + 40960011.

It is an arithmetic number, because the mean of its divisors is an integer number (40960011).

Almost surely, 2^81920021 is an apocalyptic number.

It is an amenable number.

81920021 is a deficient number, since it is larger than the sum of its proper divisors (1).

81920021 is an equidigital number, since it uses as much as digits as its factorization.

81920021 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 288, while the sum is 23.

The square root of 81920021 is about 9050.9679592848. The cubic root of 81920021 is about 434.3068557664.

Adding to 81920021 its reverse (12002918), we get a palindrome (93922939).

The spelling of 81920021 in words is "eighty-one million, nine hundred twenty thousand, twenty-one".