21555900 has 108 divisors (see below), whose sum is σ = 69261192. Its totient is φ = 5604480.

The previous prime is 21555893. The next prime is 21555917. The reversal of 21555900 is 955512.

21555900 is a `hidden beast` number, since 21 + 55 + 590 + 0 = 666.

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 35 ways as a sum of consecutive naturals, for example, 38422 + ... + 38978.

Almost surely, 2^21555900 is an apocalyptic number.

21555900 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 21555900, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (34630596).

21555900 is an abundant number, since it is smaller than the sum of its proper divisors (47705292).

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

21555900 is a wasteful number, since it uses less digits than its factorization.

With its predecessor (21555899) it forms a Ruth-Aaron pair, since the sum of their distinct prime factors is the same (610).

21555900 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 620 (or 610 counting only the distinct ones).

The product of its (nonzero) digits is 2250, while the sum is 27.

The square root of 21555900 is about 4642.8331867514. The cubic root of 21555900 is about 278.3056693395.

The spelling of 21555900 in words is "twenty-one million, five hundred fifty-five thousand, nine hundred".