21745 has 4 divisors (see below), whose sum is σ = 26100. Its totient is φ = 17392.

The previous prime is 21739. The next prime is 21751. The reversal of 21745 is 54712.

21745 is nontrivially palindromic in base 12.

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

It is an interprime number because it is at equal distance from previous prime (21739) and next prime (21751).

It can be written as a sum of positive squares in 2 ways, for example, as 3249 + 18496 = 57^2 + 136^2 .

It is a cyclic number.

It is not a de Polignac number, because 21745 - 2³ = 21737 is a prime.

It is a plaindrome in 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, 2170 + ... + 2179.

It is an arithmetic number, because the mean of its divisors is an integer number (6525).

2²¹⁷⁴⁵ is an apocalyptic number.

It is an amenable number.

21745 is a deficient number, since it is larger than the sum of its proper divisors (4355).

21745 is an equidigital number, since it uses as much as digits as its factorization.

21745 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4354.

The product of its digits is 280, while the sum is 19.

The square root of 21745 is about 147.4618594756. The cubic root of 21745 is about 27.9117117020.

It can be divided in two parts, 217 and 45, that added together give a palindrome (262).

The spelling of 21745 in words is "twenty-one thousand, seven hundred forty-five".