21811218 has 48 divisors (see below), whose sum is σ = 51659328. Its totient is φ = 6098400.

The previous prime is 21811211. The next prime is 21811261. The reversal of 21811218 is 81211812.

It is not an unprimeable number, because it can be changed into a prime (21811211) 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, 8283 + ... + 10593.

It is an arithmetic number, because the mean of its divisors is an integer number (1076236).

Almost surely, 2^21811218 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21811218, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25829664).

21811218 is an abundant number, since it is smaller than the sum of its proper divisors (29848110).

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

21811218 is a wasteful number, since it uses less digits than its factorization.

21811218 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2351 (or 2340 counting only the distinct ones).

The product of its digits is 256, while the sum is 24.

The square root of 21811218 is about 4670.2481732773. The cubic root of 21811218 is about 279.4001529336.

It can be divided in two parts, 218112 and 18, that added together give a triangular number (218130 = T₆₆₀).

The spelling of 21811218 in words is "twenty-one million, eight hundred eleven thousand, two hundred eighteen".