21121209 has 8 divisors (see below), whose sum is σ = 31290720. Its totient is φ = 14080788.

The previous prime is 21121207. The next prime is 21121211. The reversal of 21121209 is 90212112.

It is an interprime number because it is at equal distance from previous prime (21121207) and next prime (21121211).

It is not a de Polignac number, because 21121209 - 2¹ = 21121207 is a prime.

It is a super-2 number, since 2×21121209² = 892210939243362, which contains 22 as substring.

It is a junction number, because it is equal to n+sod(n) for n = 21121191 and 21121200.

It is not an unprimeable number, because it can be changed into a prime (21121201) 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, 391107 + ... + 391160.

It is an arithmetic number, because the mean of its divisors is an integer number (3911340).

Almost surely, 2^21121209 is an apocalyptic number.

It is an amenable number.

21121209 is a deficient number, since it is larger than the sum of its proper divisors (10169511).

21121209 is an equidigital number, since it uses as much as digits as its factorization.

21121209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 782276 (or 782270 counting only the distinct ones).

The product of its (nonzero) digits is 72, while the sum is 18.

The square root of 21121209 is about 4595.7816527768. The cubic root of 21121209 is about 276.4222035327.

It can be divided in two parts, 2112 and 1209, that added together give a triangular number (3321 = T₈₁).

The spelling of 21121209 in words is "twenty-one million, one hundred twenty-one thousand, two hundred nine".