21665515 has 4 divisors (see below), whose sum is σ = 25998624. Its totient is φ = 17332408.

The previous prime is 21665503. The next prime is 21665521. The reversal of 21665515 is 51556612.

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

It is a cyclic number.

It is not a de Polignac number, because 21665515 - 2¹⁹ = 21141227 is a prime.

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, 2166547 + ... + 2166556.

It is an arithmetic number, because the mean of its divisors is an integer number (6499656).

Almost surely, 2^21665515 is an apocalyptic number.

21665515 is a deficient number, since it is larger than the sum of its proper divisors (4333109).

21665515 is an equidigital number, since it uses as much as digits as its factorization.

21665515 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4333108.

The product of its digits is 9000, while the sum is 31.

The square root of 21665515 is about 4654.6229707679. The cubic root of 21665515 is about 278.7766140542.

The spelling of 21665515 in words is "twenty-one million, six hundred sixty-five thousand, five hundred fifteen".