21091680 has 144 divisors (see below), whose sum is σ = 73198944. Its totient is φ = 5529600.

The previous prime is 21091663. The next prime is 21091703. The reversal of 21091680 is 8619012.

It is a tau number, because it is divible by the number of its divisors (144).

It is a congruent number.

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, 139605 + ... + 139755.

It is an arithmetic number, because the mean of its divisors is an integer number (508326).

Almost surely, 2^21091680 is an apocalyptic number.

21091680 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 21091680, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (36599472).

21091680 is an abundant number, since it is smaller than the sum of its proper divisors (52107264).

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

21091680 is a wasteful number, since it uses less digits than its factorization.

21091680 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 269 (or 258 counting only the distinct ones).

The product of its (nonzero) digits is 864, while the sum is 27.

The square root of 21091680 is about 4592.5679091332. The cubic root of 21091680 is about 276.2933239451.

The spelling of 21091680 in words is "twenty-one million, ninety-one thousand, six hundred eighty".