21025809 has 24 divisors (see below), whose sum is σ = 35136192. Its totient is φ = 11867040.

The previous prime is 21025793. The next prime is 21025811. The reversal of 21025809 is 90852012.

It is not a de Polignac number, because 21025809 - 2⁴ = 21025793 is a prime.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21025819) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 3219 + ... + 7239.

It is an arithmetic number, because the mean of its divisors is an integer number (1464008).

Almost surely, 2^21025809 is an apocalyptic number.

It is an amenable number.

21025809 is a deficient number, since it is larger than the sum of its proper divisors (14110383).

21025809 is a wasteful number, since it uses less digits than its factorization.

21025809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4117 (or 4114 counting only the distinct ones).

The product of its (nonzero) digits is 1440, while the sum is 27.

The square root of 21025809 is about 4585.3908230379. The cubic root of 21025809 is about 276.0053952947.

The spelling of 21025809 in words is "twenty-one million, twenty-five thousand, eight hundred nine".