21001555440 has 120 divisors (see below), whose sum is σ = 70556756400. Its totient is φ = 5598308352.

The previous prime is 21001555429. The next prime is 21001555441. The reversal of 21001555440 is 4455510012.

21001555440 is a `hidden beast` number, since 2 + 100 + 1 + 5 + 554 + 4 + 0 = 666.

It is a tau number, because it is divible by the number of its divisors (120).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001555441) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 3253336 + ... + 3259784.

It is an arithmetic number, because the mean of its divisors is an integer number (587972970).

Almost surely, 2^21001555440 is an apocalyptic number.

21001555440 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21001555440 is an abundant number, since it is smaller than the sum of its proper divisors (49555200960).

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

21001555440 is a wasteful number, since it uses less digits than its factorization.

21001555440 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10991 (or 10982 counting only the distinct ones).

The product of its (nonzero) digits is 4000, while the sum is 27.

The spelling of 21001555440 in words is "twenty-one billion, one million, five hundred fifty-five thousand, four hundred forty".