21303509 has 4 divisors (see below), whose sum is σ = 21705516. Its totient is φ = 20901504.

The previous prime is 21303481. The next prime is 21303511. The reversal of 21303509 is 90530312.

It is a happy number.

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 17825284 + 3478225 = 4222^2 + 1865^2 .

It is not a de Polignac number, because 21303509 - 2¹⁶ = 21237973 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 (21303599) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 200924 + ... + 201029.

It is an arithmetic number, because the mean of its divisors is an integer number (5426379).

Almost surely, 2^21303509 is an apocalyptic number.

It is an amenable number.

21303509 is a deficient number, since it is larger than the sum of its proper divisors (402007).

21303509 is an equidigital number, since it uses as much as digits as its factorization.

21303509 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 402006.

The product of its (nonzero) digits is 810, while the sum is 23.

The square root of 21303509 is about 4615.5724455370. The cubic root of 21303509 is about 277.2152054790.

It can be divided in two parts, 21303 and 509, that added together give a palindrome (21812).

The spelling of 21303509 in words is "twenty-one million, three hundred three thousand, five hundred nine".