21759509 has 2 divisors, whose sum is σ = 21759510. Its totient is φ = 21759508.

The previous prime is 21759499. The next prime is 21759511. The reversal of 21759509 is 90595712.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20448484 + 1311025 = 4522^2 + 1145^2 .

It is a cyclic number.

It is not a de Polignac number, because 21759509 - 2⁸ = 21759253 is a prime.

It is a Sophie Germain prime.

Together with 21759511, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21759709) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10879754 + 10879755.

It is an arithmetic number, because the mean of its divisors is an integer number (10879755).

Almost surely, 2^21759509 is an apocalyptic number.

It is an amenable number.

21759509 is a deficient number, since it is larger than the sum of its proper divisors (1).

21759509 is an equidigital number, since it uses as much as digits as its factorization.

21759509 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 28350, while the sum is 38.

The square root of 21759509 is about 4664.7088869510. The cubic root of 21759509 is about 279.1791820136.

The spelling of 21759509 in words is "twenty-one million, seven hundred fifty-nine thousand, five hundred nine".