21008 has 20 divisors (see below), whose sum is σ = 44268. Its totient is φ = 9600.

The previous prime is 21001. The next prime is 21011. The reversal of 21008 is 80012.

It can be written as a sum of positive squares in 2 ways, for example, as 12544 + 8464 = 112^2 + 92^2 .

It is a nialpdrome in base 13 and base 16.

It is a self number, because there is not a number n which added to its sum of digits gives 21008.

It is not an unprimeable number, because it can be changed into a prime (21001) 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, 158 + ... + 258.

2²¹⁰⁰⁸ is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21008, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (22134).

21008 is an abundant number, since it is smaller than the sum of its proper divisors (23260).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21008 is a wasteful number, since it uses less digits than its factorization.

21008 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 122 (or 116 counting only the distinct ones).

The product of its (nonzero) digits is 16, while the sum is 11.

The square root of 21008 is about 144.9413674559. The cubic root of 21008 is about 27.5927447148.

It can be divided in two parts, 2 and 1008, that multiplied together give a triangular number (2016 = T₆₃).

The spelling of 21008 in words is "twenty-one thousand, eight".