21391609 has 4 divisors (see below), whose sum is σ = 21411316. Its totient is φ = 21371904.

The previous prime is 21391607. The next prime is 21391613. The reversal of 21391609 is 90619312.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 8410000 + 12981609 = 2900^2 + 3603^2 .

It is a cyclic number.

It is not a de Polignac number, because 21391609 - 2¹ = 21391607 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21391603) 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 3 ways as a sum of consecutive naturals, for example, 8124 + ... + 10429.

It is an arithmetic number, because the mean of its divisors is an integer number (5352829).

Almost surely, 2^21391609 is an apocalyptic number.

It is an amenable number.

21391609 is a deficient number, since it is larger than the sum of its proper divisors (19707).

21391609 is a wasteful number, since it uses less digits than its factorization.

21391609 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 19706.

The product of its (nonzero) digits is 2916, while the sum is 31.

The square root of 21391609 is about 4625.1063771550. The cubic root of 21391609 is about 277.5968182039.

The spelling of 21391609 in words is "twenty-one million, three hundred ninety-one thousand, six hundred nine".