21228521 has 4 divisors (see below), whose sum is σ = 21913344. Its totient is φ = 20543700.

The previous prime is 21228509. The next prime is 21228533. The reversal of 21228521 is 12582212.

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 an interprime number because it is at equal distance from previous prime (21228509) and next prime (21228533).

It is a de Polignac number, because none of the positive numbers 2^k-21228521 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21228421) 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, 342365 + ... + 342426.

It is an arithmetic number, because the mean of its divisors is an integer number (5478336).

Almost surely, 2^21228521 is an apocalyptic number.

It is an amenable number.

21228521 is a deficient number, since it is larger than the sum of its proper divisors (684823).

21228521 is an equidigital number, since it uses as much as digits as its factorization.

21228521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 684822.

The product of its digits is 640, while the sum is 23.

The square root of 21228521 is about 4607.4419149893. The cubic root of 21228521 is about 276.8895587736.

The spelling of 21228521 in words is "twenty-one million, two hundred twenty-eight thousand, five hundred twenty-one".