21909821 has 8 divisors (see below), whose sum is σ = 23349240. Its totient is φ = 20487168.

The previous prime is 21909817. The next prime is 21909857. The reversal of 21909821 is 12890912.

It is a happy number.

It can be written as a sum of positive squares in 4 ways, for example, as 657721 + 21252100 = 811^2 + 4610^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21909821 - 2² = 21909817 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 (21909521) 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 7 ways as a sum of consecutive naturals, for example, 1436 + ... + 6773.

It is an arithmetic number, because the mean of its divisors is an integer number (2918655).

Almost surely, 2^21909821 is an apocalyptic number.

It is an amenable number.

21909821 is a deficient number, since it is larger than the sum of its proper divisors (1439419).

21909821 is a wasteful number, since it uses less digits than its factorization.

21909821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 8383.

The product of its (nonzero) digits is 2592, while the sum is 32.

The square root of 21909821 is about 4680.7927747338. The cubic root of 21909821 is about 279.8205525366.

The spelling of 21909821 in words is "twenty-one million, nine hundred nine thousand, eight hundred twenty-one".