21205440 has 168 divisors, whose sum is σ = 75285600. Its totient is φ = 5474304.

The previous prime is 21205399. The next prime is 21205447. The reversal of 21205440 is 4450212.

21205440 is a `hidden beast` number, since 2 + 120 + 544 + 0 = 666.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21205447) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 106461 + ... + 106659.

Almost surely, 2^21205440 is an apocalyptic number.

21205440 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 21205440, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (37642800).

21205440 is an abundant number, since it is smaller than the sum of its proper divisors (54080160).

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

21205440 is a wasteful number, since it uses less digits than its factorization.

21205440 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 259 (or 246 counting only the distinct ones).

The product of its (nonzero) digits is 320, while the sum is 18.

The square root of 21205440 is about 4604.9364816466. The cubic root of 21205440 is about 276.7891717283.

Adding to 21205440 its reverse (4450212), we get a palindrome (25655652).

The spelling of 21205440 in words is "twenty-one million, two hundred five thousand, four hundred forty".