21408 has 24 divisors (see below), whose sum is σ = 56448. Its totient is φ = 7104.

The previous prime is 21407. The next prime is 21419. The reversal of 21408 is 80412.

It is a tau number, because it is divible by the number of its divisors (24).

It is not an unprimeable number, because it can be changed into a prime (21401) 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, 16 + ... + 207.

It is an arithmetic number, because the mean of its divisors is an integer number (2352).

2²¹⁴⁰⁸ 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 21408, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28224).

21408 is an abundant number, since it is smaller than the sum of its proper divisors (35040).

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

21408 is a wasteful number, since it uses less digits than its factorization.

21408 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 236 (or 228 counting only the distinct ones).

The product of its (nonzero) digits is 64, while the sum is 15.

The square root of 21408 is about 146.3147292654. The cubic root of 21408 is about 27.7667701571.

The spelling of 21408 in words is "twenty-one thousand, four hundred eight".