81019721 has 2 divisors, whose sum is σ = 81019722. Its totient is φ = 81019720.

The previous prime is 81019661. The next prime is 81019723. The reversal of 81019721 is 12791018.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 78570496 + 2449225 = 8864^2 + 1565^2 .

It is a cyclic number.

It is not a de Polignac number, because 81019721 - 2⁶ = 81019657 is a prime.

Together with 81019723, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p_4725551 and since 81019721 and 4725551 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (81019723) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40509860 + 40509861.

It is an arithmetic number, because the mean of its divisors is an integer number (40509861).

Almost surely, 2^81019721 is an apocalyptic number.

It is an amenable number.

81019721 is a deficient number, since it is larger than the sum of its proper divisors (1).

81019721 is an equidigital number, since it uses as much as digits as its factorization.

81019721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1008, while the sum is 29.

The square root of 81019721 is about 9001.0955444324. The cubic root of 81019721 is about 432.7099825685.

The spelling of 81019721 in words is "eighty-one million, nineteen thousand, seven hundred twenty-one".