21351221 has 4 divisors (see below), whose sum is σ = 22087500. Its totient is φ = 20614944.

The previous prime is 21351217. The next prime is 21351227. The reversal of 21351221 is 12215312.

It is a happy number.

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 15171025 + 6180196 = 3895^2 + 2486^2 .

It is a cyclic number.

It is not a de Polignac number, because 21351221 - 2² = 21351217 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21351221.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21351227) 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, 368096 + ... + 368153.

It is an arithmetic number, because the mean of its divisors is an integer number (5521875).

Almost surely, 2^21351221 is an apocalyptic number.

It is an amenable number.

21351221 is a deficient number, since it is larger than the sum of its proper divisors (736279).

21351221 is an equidigital number, since it uses as much as digits as its factorization.

21351221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 736278.

The product of its digits is 120, while the sum is 17.

The square root of 21351221 is about 4620.7381444960. The cubic root of 21351221 is about 277.4220044309.

Adding to 21351221 its reverse (12215312), we get a palindrome (33566533).

The spelling of 21351221 in words is "twenty-one million, three hundred fifty-one thousand, two hundred twenty-one".