21381666 has 24 divisors (see below), whose sum is σ = 43387344. Its totient is φ = 7024680.

The previous prime is 21381653. The next prime is 21381739. The reversal of 21381666 is 66618312.

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 37161 + ... + 37731.

It is an arithmetic number, because the mean of its divisors is an integer number (1807806).

Almost surely, 2^21381666 is an apocalyptic number.

21381666 is an abundant number, since it is smaller than the sum of its proper divisors (22005678).

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

21381666 is an equidigital number, since it uses as much as digits as its factorization.

21381666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 734 (or 655 counting only the distinct ones).

The product of its digits is 10368, while the sum is 33.

The square root of 21381666 is about 4624.0313580252. The cubic root of 21381666 is about 277.5538017588.

Adding to 21381666 its reverse (66618312), we get a palindrome (87999978).

The spelling of 21381666 in words is "twenty-one million, three hundred eighty-one thousand, six hundred sixty-six".