21240 has 48 divisors (see below), whose sum is σ = 70200. Its totient is φ = 5568.

The previous prime is 21227. The next prime is 21247. The reversal of 21240 is 4212.

21240 is digitally balanced in base 4, because in such base it contains all the possibile digits an equal number of times.

It is a super-2 number, since 2×21240² = 902275200, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21247) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 331 + ... + 389.

2²¹²⁴⁰ is an apocalyptic number.

21240 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 21240, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (35100).

21240 is an abundant number, since it is smaller than the sum of its proper divisors (48960).

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

21240 is a wasteful number, since it uses less digits than its factorization.

21240 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 76 (or 69 counting only the distinct ones).

The product of its (nonzero) digits is 16, while the sum is 9.

The square root of 21240 is about 145.7394936179. The cubic root of 21240 is about 27.6939457720.

Adding to 21240 its reverse (4212), we get a palindrome (25452).

It can be divided in two parts, 212 and 40, that added together give a palindrome (252).

The spelling of 21240 in words is "twenty-one thousand, two hundred forty", and thus it is an iban number.