21687840 has 72 divisors (see below), whose sum is σ = 74014668. Its totient is φ = 5783040.

The previous prime is 21687803. The next prime is 21687857. The reversal of 21687840 is 4878612.

It can be written as a sum of positive squares in 2 ways, for example, as 21566736 + 121104 = 4644^2 + 348^2 .

It is a tau number, because it is divible by the number of its divisors (72).

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

It is a junction number, because it is equal to n+sod(n) for n = 21687795 and 21687804.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 6091 + ... + 8970.

Almost surely, 2^21687840 is an apocalyptic number.

21687840 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21687840 is an abundant number, since it is smaller than the sum of its proper divisors (52326828).

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

21687840 is a wasteful number, since it uses less digits than its factorization.

21687840 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 15082 (or 15071 counting only the distinct ones).

The product of its (nonzero) digits is 21504, while the sum is 36.

The square root of 21687840 is about 4657.0205067189. The cubic root of 21687840 is about 278.8723353178.

The spelling of 21687840 in words is "twenty-one million, six hundred eighty-seven thousand, eight hundred forty".