81665 has 4 divisors (see below), whose sum is σ = 98004. Its totient is φ = 65328.

The previous prime is 81649. The next prime is 81667. The reversal of 81665 is 56618.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 56618 = 2 ⋅28309.

It can be written as a sum of positive squares in 2 ways, for example, as 16129 + 65536 = 127^2 + 256^2 .

It is a cyclic number.

It is not a de Polignac number, because 81665 - 2⁴ = 81649 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81667) 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, 8162 + ... + 8171.

It is an arithmetic number, because the mean of its divisors is an integer number (24501).

2⁸¹⁶⁶⁵ is an apocalyptic number.

It is an amenable number.

81665 is a deficient number, since it is larger than the sum of its proper divisors (16339).

81665 is a wasteful number, since it uses less digits than its factorization.

81665 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 16338.

The product of its digits is 1440, while the sum is 26.

The square root of 81665 is about 285.7708872506. The cubic root of 81665 is about 43.3855714529.

The spelling of 81665 in words is "eighty-one thousand, six hundred sixty-five".