21918221 has 4 divisors (see below), whose sum is σ = 23604252. Its totient is φ = 20232192.

The previous prime is 21918187. The next prime is 21918233. The reversal of 21918221 is 12281912.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 11485321 + 10432900 = 3389^2 + 3230^2 .

It is a cyclic number.

It is not a de Polignac number, because 21918221 - 2¹⁴ = 21901837 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 (21918251) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 842996 + ... + 843021.

It is an arithmetic number, because the mean of its divisors is an integer number (5901063).

Almost surely, 2^21918221 is an apocalyptic number.

It is an amenable number.

21918221 is a deficient number, since it is larger than the sum of its proper divisors (1686031).

21918221 is a wasteful number, since it uses less digits than its factorization.

21918221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1686030.

The product of its digits is 576, while the sum is 26.

The square root of 21918221 is about 4681.6899726488. The cubic root of 21918221 is about 279.8563080747.

The spelling of 21918221 in words is "twenty-one million, nine hundred eighteen thousand, two hundred twenty-one".