21681920 has 72 divisors (see below), whose sum is σ = 55972896. Its totient is φ = 7999488.

The previous prime is 21681911. The next prime is 21681923. The reversal of 21681920 is 2918612.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21681923) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 15989 + ... + 17291.

Almost surely, 2^21681920 is an apocalyptic number.

21681920 is a gapful number since it is divisible by the number (20) 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 21681920, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27986448).

21681920 is an abundant number, since it is smaller than the sum of its proper divisors (34290976).

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

21681920 is a wasteful number, since it uses less digits than its factorization.

21681920 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1337 (or 1323 counting only the distinct ones).

The product of its (nonzero) digits is 1728, while the sum is 29.

The square root of 21681920 is about 4656.3848638187. The cubic root of 21681920 is about 278.8469589708.

The spelling of 21681920 in words is "twenty-one million, six hundred eighty-one thousand, nine hundred twenty".