81665681 has 4 divisors (see below), whose sum is σ = 81741492. Its totient is φ = 81589872.

The previous prime is 81665677. The next prime is 81665693. The reversal of 81665681 is 18656618.

It is a happy number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 66912400 + 14753281 = 8180^2 + 3841^2 .

It is a cyclic number.

It is not a de Polignac number, because 81665681 - 2² = 81665677 is a prime.

It is a super-2 number, since 2×81665681² = 13338566906387522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81665081) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 36266 + ... + 38451.

It is an arithmetic number, because the mean of its divisors is an integer number (20435373).

Almost surely, 2^81665681 is an apocalyptic number.

It is an amenable number.

81665681 is a deficient number, since it is larger than the sum of its proper divisors (75811).

81665681 is a wasteful number, since it uses less digits than its factorization.

81665681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 75810.

The product of its digits is 69120, while the sum is 41.

The square root of 81665681 is about 9036.9066056920. The cubic root of 81665681 is about 433.8569204923.

The spelling of 81665681 in words is "eighty-one million, six hundred sixty-five thousand, six hundred eighty-one".