21505860 has 72 divisors (see below), whose sum is σ = 65736216. Its totient is φ = 5690880.

The previous prime is 21505859. The next prime is 21505889. The reversal of 21505860 is 6850512.

It can be written as a sum of positive squares in 4 ways, for example, as 492804 + 21013056 = 702^2 + 4584^2 .

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, 27880 + ... + 28640.

It is an arithmetic number, because the mean of its divisors is an integer number (913003).

Almost surely, 2^21505860 is an apocalyptic number.

21505860 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 21505860, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32868108).

21505860 is an abundant number, since it is smaller than the sum of its proper divisors (44230356).

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

21505860 is a wasteful number, since it uses less digits than its factorization.

21505860 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 933 (or 928 counting only the distinct ones).

The product of its (nonzero) digits is 2400, while the sum is 27.

The square root of 21505860 is about 4637.4411047473. The cubic root of 21505860 is about 278.0901489807.

The spelling of 21505860 in words is "twenty-one million, five hundred five thousand, eight hundred sixty".