21759 has 4 divisors (see below), whose sum is σ = 29016. Its totient is φ = 14504.

The previous prime is 21757. The next prime is 21767. The reversal of 21759 is 95712.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21759 - 2¹ = 21757 is a prime.

It is a D-number.

21759 is a lucky number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21759.

It is not an unprimeable number, because it can be changed into a prime (21751) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3624 + ... + 3629.

It is an arithmetic number, because the mean of its divisors is an integer number (7254).

2²¹⁷⁵⁹ is an apocalyptic number.

21759 is a deficient number, since it is larger than the sum of its proper divisors (7257).

21759 is an equidigital number, since it uses as much as digits as its factorization.

21759 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7256.

The product of its digits is 630, while the sum is 24.

The square root of 21759 is about 147.5093217393. The cubic root of 21759 is about 27.9177005137.

It can be divided in two parts, 217 and 59, that added together give a triangular number (276 = T₂₃).

The spelling of 21759 in words is "twenty-one thousand, seven hundred fifty-nine".