21456209 has 6 divisors (see below), whose sum is σ = 21741570. Its totient is φ = 21171920.

The previous prime is 21456203. The next prime is 21456221. The reversal of 21456209 is 90265412.

It is a happy number.

It can be written as a sum of positive squares in only one way, i.e., 15429184 + 6027025 = 3928^2 + 2455^2 .

It is not a de Polignac number, because 21456209 - 2⁸ = 21455953 is a prime.

It is a super-3 number, since 3×21456209³ (a number of 23 digits) contains 333 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21456203) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 43454 + ... + 43944.

It is an arithmetic number, because the mean of its divisors is an integer number (3623595).

Almost surely, 2^21456209 is an apocalyptic number.

It is an amenable number.

21456209 is a deficient number, since it is larger than the sum of its proper divisors (285361).

21456209 is an frugal number, since it uses more digits than its factorization.

21456209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1071 (or 580 counting only the distinct ones).

The product of its (nonzero) digits is 4320, while the sum is 29.

The square root of 21456209 is about 4632.0847358398. The cubic root of 21456209 is about 277.8759733486.

The spelling of 21456209 in words is "twenty-one million, four hundred fifty-six thousand, two hundred nine".