21739 has 2 divisors, whose sum is σ = 21740. Its totient is φ = 21738.

The previous prime is 21737. The next prime is 21751. The reversal of 21739 is 93712.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21739 - 2¹ = 21737 is a prime.

Together with 21737, it forms a pair of twin primes.

It is a Chen prime.

21739 is a lucky number.

It is not a weakly prime, because it can be changed into another prime (21737) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10869 + 10870.

It is an arithmetic number, because the mean of its divisors is an integer number (10870).

2²¹⁷³⁹ is an apocalyptic number.

21739 is a deficient number, since it is larger than the sum of its proper divisors (1).

21739 is an equidigital number, since it uses as much as digits as its factorization.

21739 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 378, while the sum is 22.

The square root of 21739 is about 147.4415138284. The cubic root of 21739 is about 27.9091442815.

It can be divided in two parts, 217 and 39, that added together give a 8-th power (256 = 2⁸).

The spelling of 21739 in words is "twenty-one thousand, seven hundred thirty-nine".