21123209 has 4 divisors (see below), whose sum is σ = 21136080. Its totient is φ = 21110340.

The previous prime is 21123187. The next prime is 21123211. The reversal of 21123209 is 90232112.

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 21123209 - 2¹² = 21119113 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21123709) 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, 3539 + ... + 7400.

It is an arithmetic number, because the mean of its divisors is an integer number (5284020).

Almost surely, 2^21123209 is an apocalyptic number.

It is an amenable number.

21123209 is a deficient number, since it is larger than the sum of its proper divisors (12871).

21123209 is a wasteful number, since it uses less digits than its factorization.

21123209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 12870.

The product of its (nonzero) digits is 216, while the sum is 20.

The square root of 21123209 is about 4595.9992384682. Note that the first 3 decimals coincide. The cubic root of 21123209 is about 276.4309282062.

It can be divided in two parts, 2 and 1123209, that added together give a palindrome (1123211).

The spelling of 21123209 in words is "twenty-one million, one hundred twenty-three thousand, two hundred nine".