21218621 has 4 divisors (see below), whose sum is σ = 21228252. Its totient is φ = 21208992.

The previous prime is 21218599. The next prime is 21218629. The reversal of 21218621 is 12681212.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 18156121 + 3062500 = 4261^2 + 1750^2 .

It is a cyclic number.

It is not a de Polignac number, because 21218621 - 2¹⁰ = 21217597 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21218629) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 305 + ... + 6521.

It is an arithmetic number, because the mean of its divisors is an integer number (5307063).

Almost surely, 2^21218621 is an apocalyptic number.

It is an amenable number.

21218621 is a deficient number, since it is larger than the sum of its proper divisors (9631).

21218621 is an equidigital number, since it uses as much as digits as its factorization.

21218621 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9630.

The product of its digits is 384, while the sum is 23.

The square root of 21218621 is about 4606.3674408367. The cubic root of 21218621 is about 276.8465092544.

Adding to 21218621 its reverse (12681212), we get a palindrome (33899833).

The spelling of 21218621 in words is "twenty-one million, two hundred eighteen thousand, six hundred twenty-one".