21555540 has 72 divisors (see below), whose sum is σ = 67511808. Its totient is φ = 5561280.

The previous prime is 21555539. The next prime is 21555553. The reversal of 21555540 is 4555512.

It is a congruent number.

It is an unprimeable number.

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, 3649 + ... + 7511.

It is an arithmetic number, because the mean of its divisors is an integer number (937664).

Almost surely, 2^21555540 is an apocalyptic number.

21555540 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 21555540, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (33755904).

21555540 is an abundant number, since it is smaller than the sum of its proper divisors (45956268).

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

21555540 is a wasteful number, since it uses less digits than its factorization.

21555540 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3909 (or 3904 counting only the distinct ones).

The product of its (nonzero) digits is 5000, while the sum is 27.

The square root of 21555540 is about 4642.7944171587. The cubic root of 21555540 is about 278.3041200251.

It can be divided in two parts, 215 and 55540, that added together give a palindrome (55755).

The spelling of 21555540 in words is "twenty-one million, five hundred fifty-five thousand, five hundred forty".