21666861 has 12 divisors (see below), whose sum is σ = 32306560. Its totient is φ = 13978440.

The previous prime is 21666829. The next prime is 21666863. The reversal of 21666861 is 16866612.

It is not a de Polignac number, because 21666861 - 2⁵ = 21666829 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21666863) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 38551 + ... + 39108.

Almost surely, 2^21666861 is an apocalyptic number.

It is an amenable number.

21666861 is a deficient number, since it is larger than the sum of its proper divisors (10639699).

21666861 is a wasteful number, since it uses less digits than its factorization.

21666861 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 77696 (or 77693 counting only the distinct ones).

The product of its digits is 20736, while the sum is 36.

The square root of 21666861 is about 4654.7675559581. The cubic root of 21666861 is about 278.7823870618.

The spelling of 21666861 in words is "twenty-one million, six hundred sixty-six thousand, eight hundred sixty-one".