81696721 has 2 divisors, whose sum is σ = 81696722. Its totient is φ = 81696720.

The previous prime is 81696701. The next prime is 81696739. The reversal of 81696721 is 12769618.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 42771600 + 38925121 = 6540^2 + 6239^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81696721 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is not a weakly prime, because it can be changed into another prime (81696701) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40848360 + 40848361.

It is an arithmetic number, because the mean of its divisors is an integer number (40848361).

Almost surely, 2^81696721 is an apocalyptic number.

It is an amenable number.

81696721 is a deficient number, since it is larger than the sum of its proper divisors (1).

81696721 is an equidigital number, since it uses as much as digits as its factorization.

81696721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 36288, while the sum is 40.

The square root of 81696721 is about 9038.6238443692. The cubic root of 81696721 is about 433.9118812086.

The spelling of 81696721 in words is "eighty-one million, six hundred ninety-six thousand, seven hundred twenty-one".