21309251 has 4 divisors (see below), whose sum is σ = 21529032. Its totient is φ = 21089472.

The previous prime is 21309221. The next prime is 21309257. The reversal of 21309251 is 15290312.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21309251 - 2¹⁴ = 21292867 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21309257) 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, 109745 + ... + 109938.

It is an arithmetic number, because the mean of its divisors is an integer number (5382258).

Almost surely, 2^21309251 is an apocalyptic number.

21309251 is a deficient number, since it is larger than the sum of its proper divisors (219781).

21309251 is an equidigital number, since it uses as much as digits as its factorization.

21309251 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 219780.

The product of its (nonzero) digits is 540, while the sum is 23.

The square root of 21309251 is about 4616.1944283143. The cubic root of 21309251 is about 277.2401094627.

Adding to 21309251 its reverse (15290312), we get a palindrome (36599563).

The spelling of 21309251 in words is "twenty-one million, three hundred nine thousand, two hundred fifty-one".