21985 has 4 divisors (see below), whose sum is σ = 26388. Its totient is φ = 17584.

The previous prime is 21977. The next prime is 21991. The reversal of 21985 is 58912.

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 9216 + 12769 = 96^2 + 113^2 .

It is a cyclic number.

It is not a de Polignac number, because 21985 - 2³ = 21977 is a prime.

It is a Duffinian number.

It is a plaindrome in base 11 and base 15.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2194 + ... + 2203.

It is an arithmetic number, because the mean of its divisors is an integer number (6597).

2²¹⁹⁸⁵ is an apocalyptic number.

It is an amenable number.

21985 is a deficient number, since it is larger than the sum of its proper divisors (4403).

21985 is an equidigital number, since it uses as much as digits as its factorization.

21985 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4402.

The product of its digits is 720, while the sum is 25.

The square root of 21985 is about 148.2733961303. The cubic root of 21985 is about 28.0140235875.

The spelling of 21985 in words is "twenty-one thousand, nine hundred eighty-five".