21009709 has 4 divisors (see below), whose sum is σ = 24011104. Its totient is φ = 18008316.

The previous prime is 21009701. The next prime is 21009719. The reversal of 21009709 is 90790012.

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 21009709 - 2³ = 21009701 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21009701) 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, 1500687 + ... + 1500700.

It is an arithmetic number, because the mean of its divisors is an integer number (6002776).

Almost surely, 2^21009709 is an apocalyptic number.

It is an amenable number.

21009709 is a deficient number, since it is larger than the sum of its proper divisors (3001395).

21009709 is an equidigital number, since it uses as much as digits as its factorization.

21009709 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3001394.

The product of its (nonzero) digits is 1134, while the sum is 28.

The square root of 21009709 is about 4583.6349112904. The cubic root of 21009709 is about 275.9349291743.

The spelling of 21009709 in words is "twenty-one million, nine thousand, seven hundred nine".