21665281 has 8 divisors (see below), whose sum is σ = 23675904. Its totient is φ = 19661520.

The previous prime is 21665279. The next prime is 21665341. The reversal of 21665281 is 18256612.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21665281 - 2¹ = 21665279 is a prime.

It is a super-2 number, since 2×21665281² = 938768801617922, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21665201) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 6720 + ... + 9406.

It is an arithmetic number, because the mean of its divisors is an integer number (2959488).

Almost surely, 2^21665281 is an apocalyptic number.

It is an amenable number.

21665281 is a deficient number, since it is larger than the sum of its proper divisors (2010623).

21665281 is a wasteful number, since it uses less digits than its factorization.

21665281 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3431.

The product of its digits is 5760, while the sum is 31.

The square root of 21665281 is about 4654.5978343999. The cubic root of 21665281 is about 278.7756104014.

It can be divided in two parts, 2166 and 5281, that added together give a palindrome (7447).

The spelling of 21665281 in words is "twenty-one million, six hundred sixty-five thousand, two hundred eighty-one".