21005640 has 192 divisors, whose sum is σ = 74692800. Its totient is φ = 5101056.

The previous prime is 21005627. The next prime is 21005651. The reversal of 21005640 is 4650012.

It is a happy number.

21005640 is a `hidden beast` number, since 2 + 100 + 564 + 0 = 666.

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

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 253039 + ... + 253121.

It is an arithmetic number, because the mean of its divisors is an integer number (389025).

Almost surely, 2^21005640 is an apocalyptic number.

21005640 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 21005640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (37346400).

21005640 is an abundant number, since it is smaller than the sum of its proper divisors (53687160).

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

21005640 is a wasteful number, since it uses less digits than its factorization.

21005640 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 156 (or 149 counting only the distinct ones).

The product of its (nonzero) digits is 240, while the sum is 18.

The square root of 21005640 is about 4583.1910280939. The cubic root of 21005640 is about 275.9171143675.

Adding to 21005640 its reverse (4650012), we get a palindrome (25655652).

The spelling of 21005640 in words is "twenty-one million, five thousand, six hundred forty".