21305 has 4 divisors (see below), whose sum is σ = 25572. Its totient is φ = 17040.

The previous prime is 21283. The next prime is 21313. The reversal of 21305 is 50312.

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 15376 + 5929 = 124^2 + 77^2 .

It is not a de Polignac number, because 21305 - 2¹² = 17209 is a prime.

It is a Duffinian number.

It is a Curzon 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, 2126 + ... + 2135.

It is an arithmetic number, because the mean of its divisors is an integer number (6393).

2²¹³⁰⁵ is an apocalyptic number.

It is an amenable number.

21305 is a deficient number, since it is larger than the sum of its proper divisors (4267).

21305 is an equidigital number, since it uses as much as digits as its factorization.

21305 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4266.

The product of its (nonzero) digits is 30, while the sum is 11.

The square root of 21305 is about 145.9623239059. The cubic root of 21305 is about 27.7221672618.

Adding to 21305 its sum of digits (11), we get a square (21316 = 146²).

Adding to 21305 its reverse (50312), we get a palindrome (71617).

The spelling of 21305 in words is "twenty-one thousand, three hundred five".