21664818 has 48 divisors (see below), whose sum is σ = 54075840. Its totient is φ = 6140448.

The previous prime is 21664813. The next prime is 21664823. The reversal of 21664818 is 81846612.

21664818 is a `hidden beast` number, since 2 + 1 + 6 + 648 + 1 + 8 = 666.

It is an interprime number because it is at equal distance from previous prime (21664813) and next prime (21664823).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21664813) 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, 16896 + ... + 18132.

It is an arithmetic number, because the mean of its divisors is an integer number (1126580).

Almost surely, 2^21664818 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21664818, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27037920).

21664818 is an abundant number, since it is smaller than the sum of its proper divisors (32411022).

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

21664818 is a wasteful number, since it uses less digits than its factorization.

21664818 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1391 (or 1388 counting only the distinct ones).

The product of its digits is 18432, while the sum is 36.

The square root of 21664818 is about 4654.5480983657. The cubic root of 21664818 is about 278.7736245201.

The spelling of 21664818 in words is "twenty-one million, six hundred sixty-four thousand, eight hundred eighteen".