21221221 has 4 divisors (see below), whose sum is σ = 24252832. Its totient is φ = 18189612.

The previous prime is 21221171. The next prime is 21221237. The reversal of 21221221 is 12212212.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21221221 - 2¹³ = 21213029 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 (21221251) 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, 1515795 + ... + 1515808.

It is an arithmetic number, because the mean of its divisors is an integer number (6063208).

Almost surely, 2^21221221 is an apocalyptic number.

It is an amenable number.

21221221 is a deficient number, since it is larger than the sum of its proper divisors (3031611).

21221221 is an equidigital number, since it uses as much as digits as its factorization.

21221221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3031610.

The product of its digits is 32, while the sum is 13.

The square root of 21221221 is about 4606.6496502339. The cubic root of 21221221 is about 276.8578164851.

Adding to 21221221 its reverse (12212212), we get a palindrome (33433433).

Subtracting from 21221221 its reverse (12212212), we obtain a palindrome (9009009).

It can be divided in two parts, 2 and 1221221, that multiplied together give a palindrome (2442442).

The spelling of 21221221 in words is "twenty-one million, two hundred twenty-one thousand, two hundred twenty-one".