21328281 has 24 divisors (see below), whose sum is σ = 33332936. Its totient is φ = 13063680.

The previous prime is 21328277. The next prime is 21328283. The reversal of 21328281 is 18282312.

It can be written as a sum of positive squares in 4 ways, for example, as 2975625 + 18352656 = 1725^2 + 4284^2 .

It is not a de Polignac number, because 21328281 - 2² = 21328277 is a prime.

It is a super-2 number, since 2×21328281² = 909791140829922, which contains 22 as substring.

It is a Smith number, since the sum of its digits (27) coincides with the sum of the digits of its prime factors.

It is not an unprimeable number, because it can be changed into a prime (21328283) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 49041 + ... + 49473.

Almost surely, 2^21328281 is an apocalyptic number.

It is an amenable number.

21328281 is a deficient number, since it is larger than the sum of its proper divisors (12004655).

21328281 is a wasteful number, since it uses less digits than its factorization.

21328281 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 873 (or 870 counting only the distinct ones).

The product of its digits is 1536, while the sum is 27.

The square root of 21328281 is about 4618.2551900041. The cubic root of 21328281 is about 277.3226136849.

It can be divided in two parts, 21328 and 281, that added together give a square (21609 = 147²).

The spelling of 21328281 in words is "twenty-one million, three hundred twenty-eight thousand, two hundred eighty-one".