21724309 has 2 divisors, whose sum is σ = 21724310. Its totient is φ = 21724308.

The previous prime is 21724303. The next prime is 21724321. The reversal of 21724309 is 90342712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20520900 + 1203409 = 4530^2 + 1097^2 .

It is a cyclic number.

It is not a de Polignac number, because 21724309 - 2⁹ = 21723797 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21724303) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10862154 + 10862155.

It is an arithmetic number, because the mean of its divisors is an integer number (10862155).

Almost surely, 2^21724309 is an apocalyptic number.

It is an amenable number.

21724309 is a deficient number, since it is larger than the sum of its proper divisors (1).

21724309 is an equidigital number, since it uses as much as digits as its factorization.

21724309 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 3024, while the sum is 28.

The square root of 21724309 is about 4660.9343483898. The cubic root of 21724309 is about 279.0285595735.

The spelling of 21724309 in words is "twenty-one million, seven hundred twenty-four thousand, three hundred nine".