21681216 has 56 divisors (see below), whose sum is σ = 63743840. Its totient is φ = 7226496.

The previous prime is 21681211. The next prime is 21681217. The reversal of 21681216 is 61218612.

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

It is a nude number because it is divisible by every one of its digits.

It is a self number, because there is not a number n which added to its sum of digits gives 21681216.

It is not an unprimeable number, because it can be changed into a prime (21681211) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 4546 + ... + 8001.

Almost surely, 2^21681216 is an apocalyptic number.

It is an amenable number.

21681216 is an abundant number, since it is smaller than the sum of its proper divisors (42062624).

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

21681216 is a wasteful number, since it uses less digits than its factorization.

21681216 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12568 (or 12552 counting only the distinct ones).

The product of its digits is 1152, while the sum is 27.

The square root of 21681216 is about 4656.3092680792. The cubic root of 21681216 is about 278.8439409358.

Adding to 21681216 its reverse (61218612), we get a palindrome (82899828).

The spelling of 21681216 in words is "twenty-one million, six hundred eighty-one thousand, two hundred sixteen".