20665 has 4 divisors (see below), whose sum is σ = 24804. Its totient is φ = 16528.

The previous prime is 20663. The next prime is 20681. The reversal of 20665 is 56602.

20665 is digitally balanced in base 6, because in such base it contains all the possibile digits an equal number of times.

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 11449 + 9216 = 107^2 + 96^2 .

It is a cyclic number.

It is not a de Polignac number, because 20665 - 2¹ = 20663 is a prime.

It is a Duffinian number.

20665 is strictly pandigital in base 6.

20665 is a lucky number.

It is a nialpdrome in base 12 and base 14.

It is not an unprimeable number, because it can be changed into a prime (20663) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2062 + ... + 2071.

It is an arithmetic number, because the mean of its divisors is an integer number (6201).

2²⁰⁶⁶⁵ is an apocalyptic number.

It is an amenable number.

20665 is a deficient number, since it is larger than the sum of its proper divisors (4139).

20665 is an equidigital number, since it uses as much as digits as its factorization.

20665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4138.

The product of its (nonzero) digits is 360, while the sum is 19.

The square root of 20665 is about 143.7532608326. The cubic root of 20665 is about 27.4417499812.

Adding to 20665 its product of nonzero digits (360), we get a square (21025 = 145²).

Subtracting 20665 from its reverse (56602), we obtain a cube (35937 = 33³).

The spelling of 20665 in words is "twenty thousand, six hundred sixty-five".