21311221 has 4 divisors (see below), whose sum is σ = 21325132. Its totient is φ = 21297312.

The previous prime is 21311219. The next prime is 21311237. The reversal of 21311221 is 12211312.

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 790321 + 20520900 = 889^2 + 4530^2 .

It is a cyclic number.

It is not a de Polignac number, because 21311221 - 2¹ = 21311219 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 (21311261) 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, 4326 + ... + 7831.

It is an arithmetic number, because the mean of its divisors is an integer number (5331283).

Almost surely, 2^21311221 is an apocalyptic number.

It is an amenable number.

21311221 is a deficient number, since it is larger than the sum of its proper divisors (13911).

21311221 is a wasteful number, since it uses less digits than its factorization.

21311221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 13910.

The product of its digits is 24, while the sum is 13.

The square root of 21311221 is about 4616.4078026102. The cubic root of 21311221 is about 277.2486526409.

Adding to 21311221 its reverse (12211312), we get a palindrome (33522533).

Subtracting from 21311221 its reverse (12211312), we obtain a palindrome (9099909).

The spelling of 21311221 in words is "twenty-one million, three hundred eleven thousand, two hundred twenty-one".