21721240 has 48 divisors (see below), whose sum is σ = 51944400. Its totient is φ = 8173056.

The previous prime is 21721229. The next prime is 21721247. The reversal of 21721240 is 4212712.

It is a happy number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21721247) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 10621 + ... + 12499.

It is an arithmetic number, because the mean of its divisors is an integer number (1082175).

Almost surely, 2^21721240 is an apocalyptic number.

21721240 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 21721240, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25972200).

21721240 is an abundant number, since it is smaller than the sum of its proper divisors (30223160).

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

21721240 is a wasteful number, since it uses less digits than its factorization.

21721240 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1924 (or 1903 counting only the distinct ones).

The product of its (nonzero) digits is 224, while the sum is 19.

The square root of 21721240 is about 4660.6051109271. The cubic root of 21721240 is about 279.0154194704.

Adding to 21721240 its reverse (4212712), we get a palindrome (25933952).

The spelling of 21721240 in words is "twenty-one million, seven hundred twenty-one thousand, two hundred forty".