21728309 has 2 divisors, whose sum is σ = 21728310. Its totient is φ = 21728308.

The previous prime is 21728299. The next prime is 21728323. The reversal of 21728309 is 90382712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20566225 + 1162084 = 4535^2 + 1078^2 .

It is a cyclic number.

It is not a de Polignac number, because 21728309 - 2⁴ = 21728293 is a prime.

It is a Sophie Germain prime.

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 (21728339) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10864154 + 10864155.

It is an arithmetic number, because the mean of its divisors is an integer number (10864155).

Almost surely, 2^21728309 is an apocalyptic number.

It is an amenable number.

21728309 is a deficient number, since it is larger than the sum of its proper divisors (1).

21728309 is an equidigital number, since it uses as much as digits as its factorization.

21728309 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 6048, while the sum is 32.

The square root of 21728309 is about 4661.3634271530. The cubic root of 21728309 is about 279.0456839500.

The spelling of 21728309 in words is "twenty-one million, seven hundred twenty-eight thousand, three hundred nine".