21681324 has 144 divisors (see below), whose sum is σ = 69521760. Its totient is φ = 5806080.

The previous prime is 21681311. The next prime is 21681347. The reversal of 21681324 is 42318612.

It is a hoax number, since the sum of its digits (27) coincides with the sum of the digits of its distinct prime factors.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is an unprimeable number.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 89844 + ... + 90084.

It is an arithmetic number, because the mean of its divisors is an integer number (482790).

Almost surely, 2^21681324 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21681324, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (34760880).

21681324 is an abundant number, since it is smaller than the sum of its proper divisors (47840436).

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

21681324 is a wasteful number, since it uses less digits than its factorization.

21681324 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 285 (or 270 counting only the distinct ones).

The product of its digits is 2304, while the sum is 27.

The square root of 21681324 is about 4656.3208652325. The cubic root of 21681324 is about 278.8444039340.

Adding to 21681324 its reverse (42318612), we get a palindrome (63999936).

The spelling of 21681324 in words is "twenty-one million, six hundred eighty-one thousand, three hundred twenty-four".