21387555 has 48 divisors (see below), whose sum is σ = 43380480. Its totient is φ = 9543744.

The previous prime is 21387511. The next prime is 21387557. The reversal of 21387555 is 55578312.

21387555 is a `hidden beast` number, since 21 + 3 + 87 + 555 = 666.

It is not a de Polignac number, because 21387555 - 2⁸ = 21387299 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21387557) 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 47 ways as a sum of consecutive naturals, for example, 12756 + ... + 14334.

It is an arithmetic number, because the mean of its divisors is an integer number (903760).

Almost surely, 2^21387555 is an apocalyptic number.

21387555 is an abundant number, since it is smaller than the sum of its proper divisors (21992925).

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

21387555 is a wasteful number, since it uses less digits than its factorization.

21387555 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1640 (or 1637 counting only the distinct ones).

The product of its digits is 42000, while the sum is 36.

The square root of 21387555 is about 4624.6680961989. The cubic root of 21387555 is about 277.5792809754.

The spelling of 21387555 in words is "twenty-one million, three hundred eighty-seven thousand, five hundred fifty-five".