21420809 has 4 divisors (see below), whose sum is σ = 22548240. Its totient is φ = 20293380.

The previous prime is 21420793. The next prime is 21420811. The reversal of 21420809 is 90802412.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21420809 - 2⁴ = 21420793 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21420859) 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, 563687 + ... + 563724.

It is an arithmetic number, because the mean of its divisors is an integer number (5637060).

Almost surely, 2^21420809 is an apocalyptic number.

It is an amenable number.

21420809 is a deficient number, since it is larger than the sum of its proper divisors (1127431).

21420809 is a wasteful number, since it uses less digits than its factorization.

21420809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1127430.

The product of its (nonzero) digits is 1152, while the sum is 26.

The square root of 21420809 is about 4628.2619848060. The cubic root of 21420809 is about 277.7230692910.

The spelling of 21420809 in words is "twenty-one million, four hundred twenty thousand, eight hundred nine".