21172909 has 4 divisors (see below), whose sum is σ = 21182140. Its totient is φ = 21163680.

The previous prime is 21172891. The next prime is 21172913. The reversal of 21172909 is 90927112.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 13527684 + 7645225 = 3678^2 + 2765^2 .

It is a cyclic number.

It is not a de Polignac number, because 21172909 - 2²³ = 12784301 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 (21172979) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1777 + ... + 6745.

It is an arithmetic number, because the mean of its divisors is an integer number (5295535).

Almost surely, 2^21172909 is an apocalyptic number.

It is an amenable number.

21172909 is a deficient number, since it is larger than the sum of its proper divisors (9231).

21172909 is an equidigital number, since it uses as much as digits as its factorization.

21172909 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9230.

The product of its (nonzero) digits is 2268, while the sum is 31.

The square root of 21172909 is about 4601.4029382353. The cubic root of 21172909 is about 276.6475596876.

The spelling of 21172909 in words is "twenty-one million, one hundred seventy-two thousand, nine hundred nine".