21978809 has 4 divisors (see below), whose sum is σ = 22054116. Its totient is φ = 21903504.

The previous prime is 21978799. The next prime is 21978871. The reversal of 21978809 is 90887912.

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 21132409 + 846400 = 4597^2 + 920^2 .

It is a cyclic number.

It is not a de Polignac number, because 21978809 - 2⁸ = 21978553 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21978809.

It is not an unprimeable number, because it can be changed into a prime (21978899) 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, 37214 + ... + 37799.

It is an arithmetic number, because the mean of its divisors is an integer number (5513529).

Almost surely, 2^21978809 is an apocalyptic number.

It is an amenable number.

21978809 is a deficient number, since it is larger than the sum of its proper divisors (75307).

21978809 is an equidigital number, since it uses as much as digits as its factorization.

21978809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 75306.

The product of its (nonzero) digits is 72576, while the sum is 44.

The square root of 21978809 is about 4688.1562473962. The cubic root of 21978809 is about 280.1139374739.

The spelling of 21978809 in words is "twenty-one million, nine hundred seventy-eight thousand, eight hundred nine".