21351609 has 18 divisors (see below), whose sum is σ = 32766110. Its totient is φ = 13395456.

The previous prime is 21351601. The next prime is 21351619. The reversal of 21351609 is 90615312.

21351609 is a `hidden beast` number, since 2 + 1 + 3 + 51 + 609 = 666.

It can be written as a sum of positive squares in 3 ways, for example, as 3686400 + 17665209 = 1920^2 + 4203^2 .

It is not a de Polignac number, because 21351609 - 2³ = 21351601 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21351601) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 1504 + ... + 6705.

Almost surely, 2^21351609 is an apocalyptic number.

It is an amenable number.

21351609 is a deficient number, since it is larger than the sum of its proper divisors (11414501).

21351609 is a wasteful number, since it uses less digits than its factorization.

21351609 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 8249 (or 8229 counting only the distinct ones).

The product of its (nonzero) digits is 1620, while the sum is 27.

The square root of 21351609 is about 4620.7801289393. The cubic root of 21351609 is about 277.4236848826.

The spelling of 21351609 in words is "twenty-one million, three hundred fifty-one thousand, six hundred nine".