21696821 has 4 divisors (see below), whose sum is σ = 21736332. Its totient is φ = 21657312.

The previous prime is 21696809. The next prime is 21696823. The reversal of 21696821 is 12869612.

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 12780625 + 8916196 = 3575^2 + 2986^2 .

It is a cyclic number.

It is not a de Polignac number, because 21696821 - 2⁶ = 21696757 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 (21696823) 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 3 ways as a sum of consecutive naturals, for example, 18920 + ... + 20033.

It is an arithmetic number, because the mean of its divisors is an integer number (5434083).

Almost surely, 2^21696821 is an apocalyptic number.

It is an amenable number.

21696821 is a deficient number, since it is larger than the sum of its proper divisors (39511).

21696821 is an equidigital number, since it uses as much as digits as its factorization.

21696821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 39510.

The product of its digits is 10368, while the sum is 35.

The square root of 21696821 is about 4657.9846500391. The cubic root of 21696821 is about 278.9108239641.

The spelling of 21696821 in words is "twenty-one million, six hundred ninety-six thousand, eight hundred twenty-one".