21309684 has 48 divisors (see below), whose sum is σ = 56609280. Its totient is φ = 6175840.

The previous prime is 21309683. The next prime is 21309689. The reversal of 21309684 is 48690312.

It is a Harshad number since it is a multiple of its sum of digits (33).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21309683) 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 15 ways as a sum of consecutive naturals, for example, 474 + ... + 6545.

It is an arithmetic number, because the mean of its divisors is an integer number (1179360).

Almost surely, 2^21309684 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 21309684, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28304640).

21309684 is an abundant number, since it is smaller than the sum of its proper divisors (35299596).

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

21309684 is a wasteful number, since it uses less digits than its factorization.

21309684 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7060 (or 7058 counting only the distinct ones).

The product of its (nonzero) digits is 10368, while the sum is 33.

The square root of 21309684 is about 4616.2413281803. The cubic root of 21309684 is about 277.2419872724.

Adding to 21309684 its reverse (48690312), we get a palindrome (69999996).

The spelling of 21309684 in words is "twenty-one million, three hundred nine thousand, six hundred eighty-four".