21018891 has 4 divisors (see below), whose sum is σ = 28025192. Its totient is φ = 14012592.

The previous prime is 21018869. The next prime is 21018929. The reversal of 21018891 is 19881012.

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

It is not a de Polignac number, because 21018891 - 2⁵ = 21018859 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21018821) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3503146 + ... + 3503151.

It is an arithmetic number, because the mean of its divisors is an integer number (7006298).

Almost surely, 2^21018891 is an apocalyptic number.

21018891 is a deficient number, since it is larger than the sum of its proper divisors (7006301).

21018891 is an equidigital number, since it uses as much as digits as its factorization.

21018891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7006300.

The product of its (nonzero) digits is 1152, while the sum is 30.

The square root of 21018891 is about 4584.6364087024. The cubic root of 21018891 is about 275.9751211559.

The spelling of 21018891 in words is "twenty-one million, eighteen thousand, eight hundred ninety-one".