21555864 has 96 divisors (see below), whose sum is σ = 67476240. Its totient is φ = 6144000.

The previous prime is 21555851. The next prime is 21555869. The reversal of 21555864 is 46855512.

21555864 is a `hidden beast` number, since 21 + 55 + 586 + 4 = 666.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21555869) by changing a digit.

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 23 ways as a sum of consecutive naturals, for example, 12664 + ... + 14264.

Almost surely, 2^21555864 is an apocalyptic number.

21555864 is a gapful number since it is divisible by the number (24) 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 21555864, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (33738120).

21555864 is an abundant number, since it is smaller than the sum of its proper divisors (45920376).

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

21555864 is a wasteful number, since it uses less digits than its factorization.

21555864 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1641 (or 1634 counting only the distinct ones).

The product of its digits is 48000, while the sum is 36.

The square root of 21555864 is about 4642.8293098067. The cubic root of 21555864 is about 278.3055144088.

The spelling of 21555864 in words is "twenty-one million, five hundred fifty-five thousand, eight hundred sixty-four".