21685666 has 4 divisors (see below), whose sum is σ = 32528502. Its totient is φ = 10842832.

The previous prime is 21685661. The next prime is 21685667. The reversal of 21685666 is 66658612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in only one way, i.e., 21669025 + 16641 = 4655^2 + 129^2 .

It is not an unprimeable number, because it can be changed into a prime (21685661) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5421415 + ... + 5421418.

Almost surely, 2^21685666 is an apocalyptic number.

21685666 is a deficient number, since it is larger than the sum of its proper divisors (10842836).

21685666 is a wasteful number, since it uses less digits than its factorization.

21685666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10842835.

The product of its digits is 103680, while the sum is 40.

The square root of 21685666 is about 4656.7870898292. The cubic root of 21685666 is about 278.8630169054.

The spelling of 21685666 in words is "twenty-one million, six hundred eighty-five thousand, six hundred sixty-six".