21651216 has 80 divisors (see below), whose sum is σ = 58579584. Its totient is φ = 6884352.

The previous prime is 21651209. The next prime is 21651221. The reversal of 21651216 is 61215612.

It is a Harshad number since it is a multiple of its sum of digits (24).

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 15 ways as a sum of consecutive naturals, for example, 129565 + ... + 129731.

Almost surely, 2^21651216 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 21651216, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29289792).

21651216 is an abundant number, since it is smaller than the sum of its proper divisors (36928368).

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

21651216 is a wasteful number, since it uses less digits than its factorization.

21651216 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 288 (or 282 counting only the distinct ones).

The product of its digits is 720, while the sum is 24.

The square root of 21651216 is about 4653.0867174382. The cubic root of 21651216 is about 278.7152707308.

Adding to 21651216 its reverse (61215612), we get a palindrome (82866828).

The spelling of 21651216 in words is "twenty-one million, six hundred fifty-one thousand, two hundred sixteen".