21020665 has 4 divisors (see below), whose sum is σ = 25224804. Its totient is φ = 16816528.

The previous prime is 21020651. The next prime is 21020683. The reversal of 21020665 is 56602012.

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 15808576 + 5212089 = 3976^2 + 2283^2 .

It is a cyclic number.

It is not a de Polignac number, because 21020665 - 2⁵ = 21020633 is a prime.

It is a Smith number, since the sum of its digits (22) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2102062 + ... + 2102071.

It is an arithmetic number, because the mean of its divisors is an integer number (6306201).

Almost surely, 2^21020665 is an apocalyptic number.

It is an amenable number.

21020665 is a deficient number, since it is larger than the sum of its proper divisors (4204139).

21020665 is an equidigital number, since it uses as much as digits as its factorization.

21020665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4204138.

The product of its (nonzero) digits is 720, while the sum is 22.

The square root of 21020665 is about 4584.8298768875. The cubic root of 21020665 is about 275.9828850621.

Adding to 21020665 its reverse (56602012), we get a palindrome (77622677).

The spelling of 21020665 in words is "twenty-one million, twenty thousand, six hundred sixty-five".