21666647 has 4 divisors (see below), whose sum is σ = 21701616. Its totient is φ = 21631680.

The previous prime is 21666643. The next prime is 21666649. The reversal of 21666647 is 74666612.

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 21666647 - 2² = 21666643 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21666647.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21666643) 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, 16538 + ... + 17799.

It is an arithmetic number, because the mean of its divisors is an integer number (5425404).

Almost surely, 2^21666647 is an apocalyptic number.

21666647 is a deficient number, since it is larger than the sum of its proper divisors (34969).

21666647 is an equidigital number, since it uses as much as digits as its factorization.

21666647 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 34968.

The product of its digits is 72576, while the sum is 38.

The square root of 21666647 is about 4654.7445687170. The cubic root of 21666647 is about 278.7814692297.

The spelling of 21666647 in words is "twenty-one million, six hundred sixty-six thousand, six hundred forty-seven".