21087840 has 48 divisors (see below), whose sum is σ = 66428208. Its totient is φ = 5623296.

The previous prime is 21087827. The next prime is 21087841. The reversal of 21087840 is 4878012.

It is a tau number, because it is divible by the number of its divisors (48).

It is a Harshad number since it is a multiple of its sum of digits (30).

It is a junction number, because it is equal to n+sod(n) for n = 21087798 and 21087807.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21087841) 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, 21487 + ... + 22446.

It is an arithmetic number, because the mean of its divisors is an integer number (1383921).

Almost surely, 2^21087840 is an apocalyptic number.

21087840 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21087840 is an abundant number, since it is smaller than the sum of its proper divisors (45340368).

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

21087840 is a wasteful number, since it uses less digits than its factorization.

21087840 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 43951 (or 43943 counting only the distinct ones).

The product of its (nonzero) digits is 3584, while the sum is 30.

The square root of 21087840 is about 4592.1498233398. The cubic root of 21087840 is about 276.2765553937.

The spelling of 21087840 in words is "twenty-one million, eighty-seven thousand, eight hundred forty".