21666556 has 6 divisors (see below), whose sum is σ = 37916480. Its totient is φ = 10833276.

The previous prime is 21666553. The next prime is 21666563. The reversal of 21666556 is 65566612.

It is a super-2 number, since 2×21666556² = 938879297802272, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21666553) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 2708316 + ... + 2708323.

Almost surely, 2^21666556 is an apocalyptic number.

It is an amenable number.

21666556 is a deficient number, since it is larger than the sum of its proper divisors (16249924).

21666556 is a wasteful number, since it uses less digits than its factorization.

21666556 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5416643 (or 5416641 counting only the distinct ones).

The product of its digits is 64800, while the sum is 37.

The square root of 21666556 is about 4654.7347937342. The cubic root of 21666556 is about 278.7810789347.

It can be divided in two parts, 21666 and 556, that added together give a palindrome (22222).

The spelling of 21666556 in words is "twenty-one million, six hundred sixty-six thousand, five hundred fifty-six".