21705 has 8 divisors (see below), whose sum is σ = 34752. Its totient is φ = 11568.

The previous prime is 21701. The next prime is 21713. The reversal of 21705 is 50712.

21705 = 42² + 43² + ... + 51².

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21705 - 2² = 21701 is a prime.

It is a super-2 number, since 2×21705² = 942214050, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (15), and also a Moran number because the ratio is a prime number: 1447 = 21705 / (2 + 1 + 7 + 0 + 5).

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 709 + ... + 738.

It is an arithmetic number, because the mean of its divisors is an integer number (4344).

2²¹⁷⁰⁵ is an apocalyptic number.

It is an amenable number.

21705 is a deficient number, since it is larger than the sum of its proper divisors (13047).

21705 is a wasteful number, since it uses less digits than its factorization.

21705 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1455.

The product of its (nonzero) digits is 70, while the sum is 15.

The square root of 21705 is about 147.3261687549. The cubic root of 21705 is about 27.8945866349.

The spelling of 21705 in words is "twenty-one thousand, seven hundred five".