21240509 has 6 divisors (see below), whose sum is σ = 21395610. Its totient is φ = 21086240.

The previous prime is 21240433. The next prime is 21240511. The reversal of 21240509 is 90504212.

It can be written as a sum of positive squares in 3 ways, for example, as 1968409 + 19272100 = 1403^2 + 4390^2 .

It is not a de Polignac number, because 21240509 - 2⁸ = 21240253 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21240529) 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, 75449 + ... + 75729.

It is an arithmetic number, because the mean of its divisors is an integer number (3565935).

Almost surely, 2^21240509 is an apocalyptic number.

It is an amenable number.

21240509 is a deficient number, since it is larger than the sum of its proper divisors (155101).

21240509 is an frugal number, since it uses more digits than its factorization.

21240509 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 831 (or 550 counting only the distinct ones).

The product of its (nonzero) digits is 720, while the sum is 23.

The square root of 21240509 is about 4608.7426701867. The cubic root of 21240509 is about 276.9416699154.

The spelling of 21240509 in words is "twenty-one million, two hundred forty thousand, five hundred nine".