21131221 has 4 divisors (see below), whose sum is σ = 22374252. Its totient is φ = 19888192.

The previous prime is 21131191. The next prime is 21131251. The reversal of 21131221 is 12213112.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21131191) and next prime (21131251).

It can be written as a sum of positive squares in 2 ways, for example, as 12215025 + 8916196 = 3495^2 + 2986^2 .

It is a cyclic number.

It is not a de Polignac number, because 21131221 - 2¹³ = 21123029 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 (21131251) 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, 621490 + ... + 621523.

It is an arithmetic number, because the mean of its divisors is an integer number (5593563).

Almost surely, 2^21131221 is an apocalyptic number.

It is an amenable number.

21131221 is a deficient number, since it is larger than the sum of its proper divisors (1243031).

21131221 is a wasteful number, since it uses less digits than its factorization.

21131221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1243030.

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

The square root of 21131221 is about 4596.8707834787. The cubic root of 21131221 is about 276.4658737273.

Adding to 21131221 its reverse (12213112), we get a palindrome (33344333).

The spelling of 21131221 in words is "twenty-one million, one hundred thirty-one thousand, two hundred twenty-one".