21445 has 4 divisors (see below), whose sum is σ = 25740. Its totient is φ = 17152.

The previous prime is 21433. The next prime is 21467. The reversal of 21445 is 54412.

21445 is nontrivially palindromic in base 4.

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 14884 + 6561 = 122^2 + 81^2 .

It is a cyclic number.

It is not a de Polignac number, because 21445 - 2⁷ = 21317 is a prime.

21445 is a lucky number.

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, 2140 + ... + 2149.

It is an arithmetic number, because the mean of its divisors is an integer number (6435).

2²¹⁴⁴⁵ is an apocalyptic number.

It is an amenable number.

21445 is a deficient number, since it is larger than the sum of its proper divisors (4295).

21445 is an equidigital number, since it uses as much as digits as its factorization.

21445 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4294.

The product of its digits is 160, while the sum is 16.

The square root of 21445 is about 146.4411144454. The cubic root of 21445 is about 27.7827576257.

Adding to 21445 its reverse (54412), we get a palindrome (75857).

The spelling of 21445 in words is "twenty-one thousand, four hundred forty-five".