21618891 has 24 divisors (see below), whose sum is σ = 36922080. Its totient is φ = 11926656.

The previous prime is 21618889. The next prime is 21618893. The reversal of 21618891 is 19881612.

It is an interprime number because it is at equal distance from previous prime (21618889) and next prime (21618893).

It is not a de Polignac number, because 21618891 - 2¹ = 21618889 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21618893) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 4090 + ... + 7743.

It is an arithmetic number, because the mean of its divisors is an integer number (1538420).

Almost surely, 2^21618891 is an apocalyptic number.

21618891 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

21618891 is a deficient number, since it is larger than the sum of its proper divisors (15303189).

21618891 is a wasteful number, since it uses less digits than its factorization.

21618891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 11875 (or 11872 counting only the distinct ones).

The product of its digits is 6912, while the sum is 36.

The square root of 21618891 is about 4649.6119192896. The cubic root of 21618891 is about 278.5764954841.

The spelling of 21618891 in words is "twenty-one million, six hundred eighteen thousand, eight hundred ninety-one".