21785 has 4 divisors (see below), whose sum is σ = 26148. Its totient is φ = 17424.

The previous prime is 21773. The next prime is 21787. The reversal of 21785 is 58712.

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

It can be written as a sum of positive squares in 2 ways, for example, as 17161 + 4624 = 131^2 + 68^2 .

It is a cyclic number.

It is not a de Polignac number, because 21785 - 2⁸ = 21529 is a prime.

It is a d-powerful number, because it can be written as 2⁸ + 1 + 7⁵ + 8⁴ + 5⁴ .

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21785.

It is not an unprimeable number, because it can be changed into a prime (21787) 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 3 ways as a sum of consecutive naturals, for example, 2174 + ... + 2183.

It is an arithmetic number, because the mean of its divisors is an integer number (6537).

2²¹⁷⁸⁵ is an apocalyptic number.

It is an amenable number.

21785 is a deficient number, since it is larger than the sum of its proper divisors (4363).

21785 is an equidigital number, since it uses as much as digits as its factorization.

21785 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4362.

The product of its digits is 560, while the sum is 23.

The square root of 21785 is about 147.5974254518. The cubic root of 21785 is about 27.9288157808.

It can be divided in two parts, 2 and 1785, that multiplied together give a triangular number (3570 = T₈₄).

The spelling of 21785 in words is "twenty-one thousand, seven hundred eighty-five".