21213540 has 72 divisors (see below), whose sum is σ = 65345280. Its totient is φ = 5569344.

The previous prime is 21213487. The next prime is 21213553. The reversal of 21213540 is 4531212.

21213540 is a `hidden beast` number, since 2 + 121 + 3 + 540 = 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 (11) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 11181 + ... + 12939.

Almost surely, 2^21213540 is an apocalyptic number.

21213540 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 21213540, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32672640).

21213540 is an abundant number, since it is smaller than the sum of its proper divisors (44131740).

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

21213540 is a wasteful number, since it uses less digits than its factorization.

21213540 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1841 (or 1836 counting only the distinct ones).

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

The square root of 21213540 is about 4605.8158886347. The cubic root of 21213540 is about 276.8244096497.

Adding to 21213540 its reverse (4531212), we get a palindrome (25744752).

The spelling of 21213540 in words is "twenty-one million, two hundred thirteen thousand, five hundred forty".