21555221 has 4 divisors (see below), whose sum is σ = 21850572. Its totient is φ = 21259872.

The previous prime is 21555211. The next prime is 21555223. The reversal of 21555221 is 12255512.

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 5354596 + 16200625 = 2314^2 + 4025^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21555221 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 (21555223) 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, 147566 + ... + 147711.

It is an arithmetic number, because the mean of its divisors is an integer number (5462643).

Almost surely, 2^21555221 is an apocalyptic number.

It is an amenable number.

21555221 is a deficient number, since it is larger than the sum of its proper divisors (295351).

21555221 is an equidigital number, since it uses as much as digits as its factorization.

21555221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 295350.

The product of its digits is 1000, while the sum is 23.

The square root of 21555221 is about 4642.7600627213. The cubic root of 21555221 is about 278.3027471460.

The spelling of 21555221 in words is "twenty-one million, five hundred fifty-five thousand, two hundred twenty-one".