21555665 has 4 divisors (see below), whose sum is σ = 25866804. Its totient is φ = 17244528.

The previous prime is 21555647. The next prime is 21555673. The reversal of 21555665 is 56655512.

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 6932689 + 14622976 = 2633^2 + 3824^2 .

It is a cyclic number.

It is not a de Polignac number, because 21555665 - 2¹⁶ = 21490129 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, 2155562 + ... + 2155571.

It is an arithmetic number, because the mean of its divisors is an integer number (6466701).

Almost surely, 2^21555665 is an apocalyptic number.

It is an amenable number.

21555665 is a deficient number, since it is larger than the sum of its proper divisors (4311139).

21555665 is an equidigital number, since it uses as much as digits as its factorization.

21555665 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4311138.

The product of its digits is 45000, while the sum is 35.

The square root of 21555665 is about 4642.8078788595. The cubic root of 21555665 is about 278.3046579834.

The spelling of 21555665 in words is "twenty-one million, five hundred fifty-five thousand, six hundred sixty-five".