21145 has 4 divisors (see below), whose sum is σ = 25380. Its totient is φ = 16912.

The previous prime is 21143. The next prime is 21149. The reversal of 21145 is 54112.

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 16384 + 4761 = 128^2 + 69^2 .

It is a cyclic number.

It is not a de Polignac number, because 21145 - 2¹ = 21143 is a prime.

It is a super-2 number, since 2×21145² = 894222050, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21143) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2110 + ... + 2119.

It is an arithmetic number, because the mean of its divisors is an integer number (6345).

2²¹¹⁴⁵ is an apocalyptic number.

It is an amenable number.

21145 is a deficient number, since it is larger than the sum of its proper divisors (4235).

21145 is an equidigital number, since it uses as much as digits as its factorization.

21145 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4234.

The product of its digits is 40, while the sum is 13.

The square root of 21145 is about 145.4132043523. The cubic root of 21145 is about 27.6525952218.

Adding to 21145 its reverse (54112), we get a palindrome (75257).

It can be divided in two parts, 211 and 45, that added together give a 8-th power (256 = 2⁸).

The spelling of 21145 in words is "twenty-one thousand, one hundred forty-five".