21512221 has 4 divisors (see below), whose sum is σ = 21521500. Its totient is φ = 21502944.

The previous prime is 21512213. The next prime is 21512263. The reversal of 21512221 is 12221512.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 6604900 + 14907321 = 2570^2 + 3861^2 .

It is a cyclic number.

It is not a de Polignac number, because 21512221 - 2³ = 21512213 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21512195 and 21512204.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21518221) 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, 2185 + ... + 6913.

It is an arithmetic number, because the mean of its divisors is an integer number (5380375).

Almost surely, 2^21512221 is an apocalyptic number.

It is an amenable number.

21512221 is a deficient number, since it is larger than the sum of its proper divisors (9279).

21512221 is an equidigital number, since it uses as much as digits as its factorization.

21512221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9278.

The product of its digits is 80, while the sum is 16.

The square root of 21512221 is about 4638.1268848534. The cubic root of 21512221 is about 278.1175640986.

Adding to 21512221 its reverse (12221512), we get a palindrome (33733733).

The spelling of 21512221 in words is "twenty-one million, five hundred twelve thousand, two hundred twenty-one".