21281 has 4 divisors (see below), whose sum is σ = 22932. Its totient is φ = 19632.

The previous prime is 21277. The next prime is 21283. The reversal of 21281 is 18212.

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 1681 + 19600 = 41^2 + 140^2 .

It is a cyclic number.

It is not a de Polignac number, because 21281 - 2² = 21277 is a prime.

It is a super-2 number, since 2×21281² = 905761922, which contains 22 as substring.

It is an Ulam number.

It is a nialpdrome in base 16.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21281.

It is not an unprimeable number, because it can be changed into a prime (21283) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 806 + ... + 831.

It is an arithmetic number, because the mean of its divisors is an integer number (5733).

2²¹²⁸¹ is an apocalyptic number.

It is an amenable number.

21281 is a deficient number, since it is larger than the sum of its proper divisors (1651).

21281 is a wasteful number, since it uses less digits than its factorization.

21281 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1650.

The product of its digits is 32, while the sum is 14.

The square root of 21281 is about 145.8800877433. The cubic root of 21281 is about 27.7117537125.

Adding to 21281 its reverse (18212), we get a palindrome (39493).

The spelling of 21281 in words is "twenty-one thousand, two hundred eighty-one".