21181221 has 12 divisors (see below), whose sum is σ = 30676620. Its totient is φ = 14083200.

The previous prime is 21181189. The next prime is 21181267. The reversal of 21181221 is 12218112.

It can be written as a sum of positive squares in 2 ways, for example, as 12852225 + 8328996 = 3585^2 + 2886^2 .

It is not a de Polignac number, because 21181221 - 2⁵ = 21181189 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21181194 and 21181203.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21185221) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 675 + ... + 6543.

It is an arithmetic number, because the mean of its divisors is an integer number (2556385).

Almost surely, 2^21181221 is an apocalyptic number.

It is an amenable number.

21181221 is a deficient number, since it is larger than the sum of its proper divisors (9495399).

21181221 is a wasteful number, since it uses less digits than its factorization.

21181221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6276 (or 6273 counting only the distinct ones).

The product of its digits is 64, while the sum is 18.

The square root of 21181221 is about 4602.3060524046. The cubic root of 21181221 is about 276.6837567869.

Adding to 21181221 its reverse (12218112), we get a palindrome (33399333).

The spelling of 21181221 in words is "twenty-one million, one hundred eighty-one thousand, two hundred twenty-one".