21003840 has 448 divisors, whose sum is σ = 92171520. Its totient is φ = 4423680.

The previous prime is 21003821. The next prime is 21003869. The reversal of 21003840 is 4830012.

It is a happy number.

It is a super-2 number, since 2×21003840² = 882322589491200, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (18).

It is an unprimeable number.

It is a polite number, since it can be written in 63 ways as a sum of consecutive naturals, for example, 1235512 + ... + 1235528.

It is an arithmetic number, because the mean of its divisors is an integer number (205740).

Almost surely, 2^21003840 is an apocalyptic number.

21003840 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 21003840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (46085760).

21003840 is an abundant number, since it is smaller than the sum of its proper divisors (71167680).

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

21003840 is a wasteful number, since it uses less digits than its factorization.

21003840 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 67 (or 51 counting only the distinct ones).

The product of its (nonzero) digits is 192, while the sum is 18.

The square root of 21003840 is about 4582.9946541535. The cubic root of 21003840 is about 275.9092329130.

Adding to 21003840 its reverse (4830012), we get a palindrome (25833852).

Subtracting from 21003840 its reverse (4830012), we obtain a triangular number (16173828 = T₅₆₈₇).

The spelling of 21003840 in words is "twenty-one million, three thousand, eight hundred forty".