21005840 has 40 divisors (see below), whose sum is σ = 49580160. Its totient is φ = 8274816.

The previous prime is 21005837. The next prime is 21005857. The reversal of 21005840 is 4850012.

It is a tau number, because it is divible by the number of its divisors (40).

It is a Harshad number since it is a multiple of its sum of digits (20).

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 3401 + ... + 7319.

It is an arithmetic number, because the mean of its divisors is an integer number (1239504).

Almost surely, 2^21005840 is an apocalyptic number.

21005840 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 21005840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (24790080).

21005840 is an abundant number, since it is smaller than the sum of its proper divisors (28574320).

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

21005840 is a wasteful number, since it uses less digits than its factorization.

21005840 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3999 (or 3993 counting only the distinct ones).

The product of its (nonzero) digits is 320, while the sum is 20.

The square root of 21005840 is about 4583.2128469012. The cubic root of 21005840 is about 275.9179900569.

Adding to 21005840 its reverse (4850012), we get a palindrome (25855852).

The spelling of 21005840 in words is "twenty-one million, five thousand, eight hundred forty".