21519809 has 4 divisors (see below), whose sum is σ = 21584004. Its totient is φ = 21455616.

The previous prime is 21519803. The next prime is 21519811. The reversal of 21519809 is 90891512.

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 10061584 + 11458225 = 3172^2 + 3385^2 .

It is a cyclic number.

It is not a de Polignac number, because 21519809 - 2²⁰ = 20471233 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21519803) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 31592 + ... + 32265.

It is an arithmetic number, because the mean of its divisors is an integer number (5396001).

Almost surely, 2^21519809 is an apocalyptic number.

It is an amenable number.

21519809 is a deficient number, since it is larger than the sum of its proper divisors (64195).

21519809 is an equidigital number, since it uses as much as digits as its factorization.

21519809 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 64194.

The product of its (nonzero) digits is 6480, while the sum is 35.

The square root of 21519809 is about 4638.9448153648. The cubic root of 21519809 is about 278.1502603661.

The spelling of 21519809 in words is "twenty-one million, five hundred nineteen thousand, eight hundred nine".