21721216 has 32 divisors (see below), whose sum is σ = 47209680. Its totient is φ = 9872640.

The previous prime is 21721211. The next prime is 21721229. The reversal of 21721216 is 61212712.

It is a happy number.

It is a tau number, because it is divible by the number of its divisors (32).

It is a super-2 number, since 2×21721216² = 943622449037312, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (22).

It is not an unprimeable number, because it can be changed into a prime (21721211) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 6306 + ... + 9121.

Almost surely, 2^21721216 is an apocalyptic number.

It is an amenable number.

21721216 is an abundant number, since it is smaller than the sum of its proper divisors (25488464).

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

21721216 is a wasteful number, since it uses less digits than its factorization.

21721216 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 15452 (or 15440 counting only the distinct ones).

The product of its digits is 336, while the sum is 22.

The square root of 21721216 is about 4660.6025361535. The cubic root of 21721216 is about 279.0153167081.

Adding to 21721216 its reverse (61212712), we get a palindrome (82933928).

The spelling of 21721216 in words is "twenty-one million, seven hundred twenty-one thousand, two hundred sixteen".