21245809 has 4 divisors (see below), whose sum is σ = 22880116. Its totient is φ = 19611504.

The previous prime is 21245803. The next prime is 21245839. The reversal of 21245809 is 90854212.

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 6615184 + 14630625 = 2572^2 + 3825^2 .

It is a cyclic number.

It is not a de Polignac number, because 21245809 - 2⁹ = 21245297 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21245803) 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, 817134 + ... + 817159.

It is an arithmetic number, because the mean of its divisors is an integer number (5720029).

Almost surely, 2^21245809 is an apocalyptic number.

It is an amenable number.

21245809 is a deficient number, since it is larger than the sum of its proper divisors (1634307).

21245809 is a wasteful number, since it uses less digits than its factorization.

21245809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1634306.

The product of its (nonzero) digits is 5760, while the sum is 31.

The square root of 21245809 is about 4609.3176284565. The cubic root of 21245809 is about 276.9647024579.

The spelling of 21245809 in words is "twenty-one million, two hundred forty-five thousand, eight hundred nine".