21561809 has 4 divisors (see below), whose sum is σ = 21583716. Its totient is φ = 21539904.

The previous prime is 21561781. The next prime is 21561817. The reversal of 21561809 is 90816512.

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 15171025 + 6390784 = 3895^2 + 2528^2 .

It is a cyclic number.

It is not a de Polignac number, because 21561809 - 2¹⁶ = 21496273 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21560809) 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, 9404 + ... + 11469.

It is an arithmetic number, because the mean of its divisors is an integer number (5395929).

Almost surely, 2^21561809 is an apocalyptic number.

It is an amenable number.

21561809 is a deficient number, since it is larger than the sum of its proper divisors (21907).

21561809 is a wasteful number, since it uses less digits than its factorization.

21561809 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 21906.

The product of its (nonzero) digits is 4320, while the sum is 32.

The square root of 21561809 is about 4643.4695002767. The cubic root of 21561809 is about 278.3310971490.

The spelling of 21561809 in words is "twenty-one million, five hundred sixty-one thousand, eight hundred nine".