21640240 has 80 divisors (see below), whose sum is σ = 55353600. Its totient is φ = 7831296.

The previous prime is 21640237. The next prime is 21640249. The reversal of 21640240 is 4204612.

It is a tau number, because it is divible by the number of its divisors (80).

It is a Harshad number since it is a multiple of its sum of digits (19).

It is not an unprimeable number, because it can be changed into a prime (21640249) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 34651 + ... + 35269.

It is an arithmetic number, because the mean of its divisors is an integer number (691920).

Almost surely, 2^21640240 is an apocalyptic number.

21640240 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 21640240, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27676800).

21640240 is an abundant number, since it is smaller than the sum of its proper divisors (33713360).

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

21640240 is a wasteful number, since it uses less digits than its factorization.

21640240 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 674 (or 668 counting only the distinct ones).

The product of its (nonzero) digits is 384, while the sum is 19.

The square root of 21640240 is about 4651.9071357885. The cubic root of 21640240 is about 278.6681648946.

Adding to 21640240 its reverse (4204612), we get a palindrome (25844852).

The spelling of 21640240 in words is "twenty-one million, six hundred forty thousand, two hundred forty".