21818 has 4 divisors (see below), whose sum is σ = 32730.
Its totient is φ = 10908.
The previous prime is 21817. The next prime is 21821. The reversal of 21818 is 81812.
21818 = 372 + 382 + ... + 482.
It is a semiprime because it is the product of two primes.
It can be written as a sum of positive squares in only one way, i.e., 20449 + 1369 = 143^2 + 37^2
It is an alternating number because its digits alternate between even and odd.
21818 is a modest number, since divided by 1818 gives 2 as remainder.
It is a self number, because there is not a number n which added to its sum of digits gives 21818.
It is not an unprimeable number, because it can be changed into a prime (21817) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5453 + ... + 5456.
221818 is an apocalyptic number.
21818 is a deficient number, since it is larger than the sum of its proper divisors (10912).
21818 is a wasteful number, since it uses less digits than its factorization.
21818 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 10911.
The product of its digits is 128, while the sum is 20.
The square root of 21818 is about 147.7091737165.
The cubic root of 21818 is about 27.9429108912.
The spelling of 21818 in words is "twenty-one thousand, eight hundred eighteen".