21891661866 has 12 divisors (see below), whose sum is σ = 47431934082. Its totient is φ = 7297220616.

The previous prime is 21891661861. The next prime is 21891661879. The reversal of 21891661866 is 66816619812.

21891661866 is a `hidden beast` number, since 2 + 1 + 8 + 9 + 16 + 618 + 6 + 6 = 666.

It can be written as a sum of positive squares in only one way, i.e., 15007475025 + 6884186841 = 122505^2 + 82971^2 .

It is not an unprimeable number, because it can be changed into a prime (21891661861) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 608101701 + ... + 608101736.

Almost surely, 2^21891661866 is an apocalyptic number.

21891661866 is an abundant number, since it is smaller than the sum of its proper divisors (25540272216).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21891661866 is a wasteful number, since it uses less digits than its factorization.

21891661866 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1216203445 (or 1216203442 counting only the distinct ones).

The product of its digits is 1492992, while the sum is 54.

The spelling of 21891661866 in words is "twenty-one billion, eight hundred ninety-one million, six hundred sixty-one thousand, eight hundred sixty-six".