21991680 has 216 divisors, whose sum is σ = 80353728. Its totient is φ = 5541888.

The previous prime is 21991649. The next prime is 21991691. The reversal of 21991680 is 8619912.

It is a Harshad number since it is a multiple of its sum of digits (36).

It is an unprimeable number.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 264919 + ... + 265001.

It is an arithmetic number, because the mean of its divisors is an integer number (372008).

Almost surely, 2^21991680 is an apocalyptic number.

21991680 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 21991680, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (40176864).

21991680 is an abundant number, since it is smaller than the sum of its proper divisors (58362048).

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

21991680 is a wasteful number, since it uses less digits than its factorization.

21991680 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 133 (or 116 counting only the distinct ones).

The product of its (nonzero) digits is 7776, while the sum is 36.

The square root of 21991680 is about 4689.5287609738. The cubic root of 21991680 is about 280.1686059342.

The spelling of 21991680 in words is "twenty-one million, nine hundred ninety-one thousand, six hundred eighty".