81661721 has 2 divisors, whose sum is σ = 81661722. Its totient is φ = 81661720.

The previous prime is 81661703. The next prime is 81661733. The reversal of 81661721 is 12716618.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 80371225 + 1290496 = 8965^2 + 1136^2 .

It is a cyclic number.

It is not a de Polignac number, because 81661721 - 2²² = 77467417 is a prime.

It is not a weakly prime, because it can be changed into another prime (81661751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40830860 + 40830861.

It is an arithmetic number, because the mean of its divisors is an integer number (40830861).

Almost surely, 2^81661721 is an apocalyptic number.

It is an amenable number.

81661721 is a deficient number, since it is larger than the sum of its proper divisors (1).

81661721 is an equidigital number, since it uses as much as digits as its factorization.

81661721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4032, while the sum is 32.

The square root of 81661721 is about 9036.6875015129. The cubic root of 81661721 is about 433.8499077498.

The spelling of 81661721 in words is "eighty-one million, six hundred sixty-one thousand, seven hundred twenty-one".