21618 has 12 divisors (see below), whose sum is σ = 46878. Its totient is φ = 7200.

The previous prime is 21617. The next prime is 21647. The reversal of 21618 is 81612.

21618 is an esthetic number in base 13, because in such base its adjacent digits differ by 1.

It can be written as a sum of positive squares in only one way, i.e., 21609 + 9 = 147^2 + 3^2 .

It is a Harshad number since it is a multiple of its sum of digits (18), and also a Moran number because the ratio is a prime number: 1201 = 21618 / (2 + 1 + 6 + 1 + 8).

It is an alternating number because its digits alternate between even and odd.

It is a Curzon number.

It is a plaindrome in base 13.

It is a self number, because there is not a number n which added to its sum of digits gives 21618.

It is not an unprimeable number, because it can be changed into a prime (21611) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 583 + ... + 618.

2²¹⁶¹⁸ is an apocalyptic number.

21618 is an abundant number, since it is smaller than the sum of its proper divisors (25260).

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

21618 is a wasteful number, since it uses less digits than its factorization.

21618 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1209 (or 1206 counting only the distinct ones).

The product of its digits is 96, while the sum is 18.

The square root of 21618 is about 147.0306090581. The cubic root of 21618 is about 27.8572668352.

The spelling of 21618 in words is "twenty-one thousand, six hundred eighteen".