21705839 has 2 divisors, whose sum is σ = 21705840. Its totient is φ = 21705838.

The previous prime is 21705829. The next prime is 21705863. The reversal of 21705839 is 93850712.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21705839 is a prime.

It is a super-2 number, since 2×21705839² = 942286893387842, which contains 22 as substring.

It is a Chen prime.

It is a junction number, because it is equal to n+sod(n) for n = 21705799 and 21705808.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21705829) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10852919 + 10852920.

It is an arithmetic number, because the mean of its divisors is an integer number (10852920).

Almost surely, 2^21705839 is an apocalyptic number.

21705839 is a deficient number, since it is larger than the sum of its proper divisors (1).

21705839 is an equidigital number, since it uses as much as digits as its factorization.

21705839 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 15120, while the sum is 35.

The square root of 21705839 is about 4658.9525646866. The cubic root of 21705839 is about 278.9494604910.

The spelling of 21705839 in words is "twenty-one million, seven hundred five thousand, eight hundred thirty-nine".