21405665 has 4 divisors (see below), whose sum is σ = 25686804. Its totient is φ = 17124528.

The previous prime is 21405661. The next prime is 21405667. The reversal of 21405665 is 56650412.

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 9784384 + 11621281 = 3128^2 + 3409^2 .

It is a cyclic number.

It is not a de Polignac number, because 21405665 - 2² = 21405661 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21405661) 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, 2140562 + ... + 2140571.

It is an arithmetic number, because the mean of its divisors is an integer number (6421701).

Almost surely, 2^21405665 is an apocalyptic number.

It is an amenable number.

21405665 is a deficient number, since it is larger than the sum of its proper divisors (4281139).

21405665 is an equidigital number, since it uses as much as digits as its factorization.

21405665 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4281138.

The product of its (nonzero) digits is 7200, while the sum is 29.

The square root of 21405665 is about 4626.6256602409. The cubic root of 21405665 is about 277.6576060042.

The spelling of 21405665 in words is "twenty-one million, four hundred five thousand, six hundred sixty-five".