21508 has 12 divisors (see below), whose sum is σ = 39760. Its totient is φ = 10152.

The previous prime is 21503. The next prime is 21517. The reversal of 21508 is 80512.

It is a happy number.

It is a junction number, because it is equal to n+sod(n) for n = 21491 and 21500.

It is not an unprimeable number, because it can be changed into a prime (21503) 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, 66 + ... + 217.

2²¹⁵⁰⁸ is an apocalyptic number.

It is an amenable number.

21508 is a deficient number, since it is larger than the sum of its proper divisors (18252).

21508 is a wasteful number, since it uses less digits than its factorization.

21508 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 306 (or 304 counting only the distinct ones).

The product of its (nonzero) digits is 80, while the sum is 16.

The square root of 21508 is about 146.6560602225. The cubic root of 21508 is about 27.8099372718.

It can be divided in two parts, 21 and 508, that added together give a square (529 = 23²).

The spelling of 21508 in words is "twenty-one thousand, five hundred eight".