21706639 has 2 divisors, whose sum is σ = 21706640. Its totient is φ = 21706638.

The previous prime is 21706609. The next prime is 21706693. The reversal of 21706639 is 93660712.

Together with next prime (21706693) it forms an Ormiston pair, because they use the same digits, order apart.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21706639 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21706609) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10853319 + 10853320.

It is an arithmetic number, because the mean of its divisors is an integer number (10853320).

Almost surely, 2^21706639 is an apocalyptic number.

21706639 is a deficient number, since it is larger than the sum of its proper divisors (1).

21706639 is an equidigital number, since it uses as much as digits as its factorization.

21706639 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 13608, while the sum is 34.

The square root of 21706639 is about 4659.0384201034. The cubic root of 21706639 is about 278.9528874771.

The spelling of 21706639 in words is "twenty-one million, seven hundred six thousand, six hundred thirty-nine".