21660021 has 16 divisors (see below), whose sum is σ = 32635200. Its totient is φ = 14194224.

The previous prime is 21660011. The next prime is 21660047. The reversal of 21660021 is 12006612.

It is a happy number.

21660021 is a `hidden beast` number, since 2 + 1 + 660 + 0 + 2 + 1 = 666.

It is a de Polignac number, because none of the positive numbers 2^k-21660021 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21659976 and 21660003.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21660011) by changing a digit.

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 15 ways as a sum of consecutive naturals, for example, 5206 + ... + 8391.

It is an arithmetic number, because the mean of its divisors is an integer number (2039700).

Almost surely, 2^21660021 is an apocalyptic number.

It is an amenable number.

21660021 is a deficient number, since it is larger than the sum of its proper divisors (10975179).

21660021 is a wasteful number, since it uses less digits than its factorization.

21660021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 13665 (or 13659 counting only the distinct ones).

The product of its (nonzero) digits is 144, while the sum is 18.

The square root of 21660021 is about 4654.0327673965. The cubic root of 21660021 is about 278.7530477523.

Adding to 21660021 its reverse (12006612), we get a palindrome (33666633).

The spelling of 21660021 in words is "twenty-one million, six hundred sixty thousand, twenty-one".