21724517 has 2 divisors, whose sum is σ = 21724518. Its totient is φ = 21724516.

The previous prime is 21724499. The next prime is 21724519. The reversal of 21724517 is 71542712.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21242881 + 481636 = 4609^2 + 694^2 .

It is a cyclic number.

It is not a de Polignac number, because 21724517 - 2²⁴ = 4947301 is a prime.

Together with 21724519, it forms a pair of twin primes.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21724517.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21724519) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10862258 + 10862259.

It is an arithmetic number, because the mean of its divisors is an integer number (10862259).

Almost surely, 2^21724517 is an apocalyptic number.

It is an amenable number.

21724517 is a deficient number, since it is larger than the sum of its proper divisors (1).

21724517 is an equidigital number, since it uses as much as digits as its factorization.

21724517 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3920, while the sum is 29.

The square root of 21724517 is about 4660.9566614591. The cubic root of 21724517 is about 279.0294500929.

The spelling of 21724517 in words is "twenty-one million, seven hundred twenty-four thousand, five hundred seventeen".