21355309 has 4 divisors (see below), whose sum is σ = 21656160. Its totient is φ = 21054460.

The previous prime is 21355291. The next prime is 21355351. The reversal of 21355309 is 90355312.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21355309 - 2⁵ = 21355277 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 (21355409) 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, 150319 + ... + 150460.

It is an arithmetic number, because the mean of its divisors is an integer number (5414040).

Almost surely, 2^21355309 is an apocalyptic number.

It is an amenable number.

21355309 is a deficient number, since it is larger than the sum of its proper divisors (300851).

21355309 is an equidigital number, since it uses as much as digits as its factorization.

21355309 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 300850.

The product of its (nonzero) digits is 4050, while the sum is 28.

The square root of 21355309 is about 4621.1804768912. The cubic root of 21355309 is about 277.4397087858.

The spelling of 21355309 in words is "twenty-one million, three hundred fifty-five thousand, three hundred nine".