21968521 has 4 divisors (see below), whose sum is σ = 22007700. Its totient is φ = 21929344.

The previous prime is 21968509. The next prime is 21968543. The reversal of 21968521 is 12586912.

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

It can be written as a sum of positive squares in 2 ways, for example, as 4862025 + 17106496 = 2205^2 + 4136^2 .

It is a cyclic number.

It is not a de Polignac number, because 21968521 - 2⁵ = 21968489 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21968321) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 18736 + ... + 19873.

It is an arithmetic number, because the mean of its divisors is an integer number (5501925).

Almost surely, 2^21968521 is an apocalyptic number.

It is an amenable number.

21968521 is a deficient number, since it is larger than the sum of its proper divisors (39179).

21968521 is an equidigital number, since it uses as much as digits as its factorization.

21968521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 39178.

The product of its digits is 8640, while the sum is 34.

The square root of 21968521 is about 4687.0588859113. The cubic root of 21968521 is about 280.0702247329.

The spelling of 21968521 in words is "twenty-one million, nine hundred sixty-eight thousand, five hundred twenty-one".