21206665 has 8 divisors (see below), whose sum is σ = 25879680. Its totient is φ = 16677552.

The previous prime is 21206657. The next prime is 21206677. The reversal of 21206665 is 56660212.

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 21206665 - 2³ = 21206657 is a prime.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 35649 + ... + 36238.

It is an arithmetic number, because the mean of its divisors is an integer number (3234960).

Almost surely, 2^21206665 is an apocalyptic number.

It is an amenable number.

21206665 is a deficient number, since it is larger than the sum of its proper divisors (4673015).

21206665 is an equidigital number, since it uses as much as digits as its factorization.

21206665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 71951.

The product of its (nonzero) digits is 4320, while the sum is 28.

The square root of 21206665 is about 4605.0694891608. The cubic root of 21206665 is about 276.7945014960.

Adding to 21206665 its reverse (56660212), we get a palindrome (77866877).

The spelling of 21206665 in words is "twenty-one million, two hundred six thousand, six hundred sixty-five".