21007909 has 8 divisors (see below), whose sum is σ = 22757112. Its totient is φ = 19277760.

The previous prime is 21007891. The next prime is 21007913. The reversal of 21007909 is 90970012.

It can be written as a sum of positive squares in 4 ways, for example, as 2259009 + 18748900 = 1503^2 + 4330^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21007909 - 2⁷ = 21007781 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21007999) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2422 + ... + 6919.

It is an arithmetic number, because the mean of its divisors is an integer number (2844639).

Almost surely, 2^21007909 is an apocalyptic number.

It is an amenable number.

21007909 is a deficient number, since it is larger than the sum of its proper divisors (1749203).

21007909 is a wasteful number, since it uses less digits than its factorization.

21007909 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9527.

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

The square root of 21007909 is about 4583.4385563679. The cubic root of 21007909 is about 275.9270487374.

The spelling of 21007909 in words is "twenty-one million, seven thousand, nine hundred nine".