21687720 has 32 divisors (see below), whose sum is σ = 65063520. Its totient is φ = 5783360.

The previous prime is 21687719. The next prime is 21687749. The reversal of 21687720 is 2778612.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 90246 + ... + 90485.

It is an arithmetic number, because the mean of its divisors is an integer number (2033235).

Almost surely, 2^21687720 is an apocalyptic number.

21687720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21687720 is an abundant number, since it is smaller than the sum of its proper divisors (43375800).

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

21687720 is a wasteful number, since it uses less digits than its factorization.

21687720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 180745 (or 180741 counting only the distinct ones).

The product of its (nonzero) digits is 9408, while the sum is 33.

The square root of 21687720 is about 4657.0076229270. The cubic root of 21687720 is about 278.8718209783.

The spelling of 21687720 in words is "twenty-one million, six hundred eighty-seven thousand, seven hundred twenty".