21205 has 4 divisors (see below), whose sum is σ = 25452. Its totient is φ = 16960.

The previous prime is 21193. The next prime is 21211. The reversal of 21205 is 50212.

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 15876 + 5329 = 126^2 + 73^2 .

It is not a de Polignac number, because 21205 - 2⁹ = 20693 is a prime.

It is a Duffinian number.

21205 is a lucky number.

It is a nialpdrome in base 13.

It is a junction number, because it is equal to n+sod(n) for n = 21191 and 21200.

It is a congruent number.

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, 2116 + ... + 2125.

It is an arithmetic number, because the mean of its divisors is an integer number (6363).

2²¹²⁰⁵ is an apocalyptic number.

It is an amenable number.

21205 is a deficient number, since it is larger than the sum of its proper divisors (4247).

21205 is an equidigital number, since it uses as much as digits as its factorization.

21205 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4246.

The product of its (nonzero) digits is 20, while the sum is 10.

The square root of 21205 is about 145.6193668438. The cubic root of 21205 is about 27.6787257313.

Adding to 21205 its reverse (50212), we get a palindrome (71417).

The spelling of 21205 in words is "twenty-one thousand, two hundred five".