21208709 has 2 divisors, whose sum is σ = 21208710. Its totient is φ = 21208708.

The previous prime is 21208699. The next prime is 21208711. The reversal of 21208709 is 90780212.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12623809 + 8584900 = 3553^2 + 2930^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21208709 is a prime.

Together with 21208711, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21208729) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10604354 + 10604355.

It is an arithmetic number, because the mean of its divisors is an integer number (10604355).

Almost surely, 2^21208709 is an apocalyptic number.

It is an amenable number.

21208709 is a deficient number, since it is larger than the sum of its proper divisors (1).

21208709 is an equidigital number, since it uses as much as digits as its factorization.

21208709 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2016, while the sum is 29.

The square root of 21208709 is about 4605.2914131464. The cubic root of 21208709 is about 276.8033941371.

The spelling of 21208709 in words is "twenty-one million, two hundred eight thousand, seven hundred nine".