21981680 has 80 divisors (see below), whose sum is σ = 61871040. Its totient is φ = 7090176.

The previous prime is 21981679. The next prime is 21981683. The reversal of 21981680 is 8618912.

It is a tau number, because it is divible by the number of its divisors (80).

It is a Harshad number since it is a multiple of its sum of digits (35).

It is a self number, because there is not a number n which added to its sum of digits gives 21981680.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21981683) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 8366 + ... + 10674.

It is an arithmetic number, because the mean of its divisors is an integer number (773388).

Almost surely, 2^21981680 is an apocalyptic number.

21981680 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 21981680, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30935520).

21981680 is an abundant number, since it is smaller than the sum of its proper divisors (39889360).

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

21981680 is a wasteful number, since it uses less digits than its factorization.

21981680 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2346 (or 2340 counting only the distinct ones).

The product of its (nonzero) digits is 6912, while the sum is 35.

The square root of 21981680 is about 4688.4624345301. The cubic root of 21981680 is about 280.1261336473.

The spelling of 21981680 in words is "twenty-one million, nine hundred eighty-one thousand, six hundred eighty".