21005 has 4 divisors (see below), whose sum is σ = 25212. Its totient is φ = 16800.

The previous prime is 21001. The next prime is 21011. The reversal of 21005 is 50012.

21005 is nontrivially palindromic in base 8.

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 3844 + 17161 = 62^2 + 131^2 .

It is not a de Polignac number, because 21005 - 2² = 21001 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 20983 and 21001.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2096 + ... + 2105.

It is an arithmetic number, because the mean of its divisors is an integer number (6303).

2²¹⁰⁰⁵ is an apocalyptic number.

It is an amenable number.

21005 is a deficient number, since it is larger than the sum of its proper divisors (4207).

21005 is an equidigital number, since it uses as much as digits as its factorization.

21005 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4206.

The product of its (nonzero) digits is 10, while the sum is 8.

The square root of 21005 is about 144.9310180741. The cubic root of 21005 is about 27.5914312124.

Adding to 21005 its reverse (50012), we get a palindrome (71017).

The spelling of 21005 in words is "twenty-one thousand, five".