21005544 has 64 divisors (see below), whose sum is σ = 60681600. Its totient is φ = 5935104.

The previous prime is 21005539. The next prime is 21005561. The reversal of 21005544 is 44550012.

It is a Harshad number since it is a multiple of its sum of digits (21).

It is a self number, because there is not a number n which added to its sum of digits gives 21005544.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 15652 + ... + 16940.

It is an arithmetic number, because the mean of its divisors is an integer number (948150).

Almost surely, 2^21005544 is an apocalyptic number.

21005544 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 21005544, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30340800).

21005544 is an abundant number, since it is smaller than the sum of its proper divisors (39676056).

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

21005544 is a wasteful number, since it uses less digits than its factorization.

21005544 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1402 (or 1398 counting only the distinct ones).

The product of its (nonzero) digits is 800, while the sum is 21.

The square root of 21005544 is about 4583.1805550294. The cubic root of 21005544 is about 275.9166940347.

Adding to 21005544 its reverse (44550012), we get a palindrome (65555556).

The spelling of 21005544 in words is "twenty-one million, five thousand, five hundred forty-four".