21721666 has 16 divisors (see below), whose sum is σ = 33136128. Its totient is φ = 10678000.

The previous prime is 21721621. The next prime is 21721717. The reversal of 21721666 is 66612712.

It is a happy number.

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, 38301 + ... + 38863.

It is an arithmetic number, because the mean of its divisors is an integer number (2071008).

Almost surely, 2^21721666 is an apocalyptic number.

21721666 is a deficient number, since it is larger than the sum of its proper divisors (11414462).

21721666 is a wasteful number, since it uses less digits than its factorization.

21721666 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 857.

The product of its digits is 6048, while the sum is 31.

The square root of 21721666 is about 4660.6508129230. The cubic root of 21721666 is about 279.0172434883.

The spelling of 21721666 in words is "twenty-one million, seven hundred twenty-one thousand, six hundred sixty-six".