21681408 has 144 divisors (see below), whose sum is σ = 68351360. Its totient is φ = 5971968.

The previous prime is 21681389. The next prime is 21681421. The reversal of 21681408 is 80418612.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 198858 + ... + 198966.

Almost surely, 2^21681408 is an apocalyptic number.

21681408 is a gapful number since it is divisible by the number (28) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21681408, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (34175680).

21681408 is an abundant number, since it is smaller than the sum of its proper divisors (46669952).

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

21681408 is a wasteful number, since it uses less digits than its factorization.

21681408 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 172 (or 158 counting only the distinct ones).

The product of its (nonzero) digits is 3072, while the sum is 30.

The square root of 21681408 is about 4656.3298852208. The cubic root of 21681408 is about 278.8447640427.

Subtracting from 21681408 its product of nonzero digits (3072), we obtain a square (21678336 = 4656²).

The spelling of 21681408 in words is "twenty-one million, six hundred eighty-one thousand, four hundred eight".