21216 has 48 divisors (see below), whose sum is σ = 63504. Its totient is φ = 6144.

The previous prime is 21211. The next prime is 21221. The reversal of 21216 is 61212.

21216 is nontrivially palindromic in base 15.

It is an interprime number because it is at equal distance from previous prime (21211) and next prime (21221).

It is a tau number, because it is divible by the number of its divisors (48).

It is a Harshad number since it is a multiple of its sum of digits (12).

It is a nude number because it is divisible by every one of its digits and also a Zuckerman number because it is divisible by the product of its digits.

It is an alternating number because its digits alternate between even and odd.

It is a plaindrome in base 7.

It is a nialpdrome in base 13.

It is a zygodrome in base 7.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21211) by changing a digit.

21216 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1240 + ... + 1256.

It is an arithmetic number, because the mean of its divisors is an integer number (1323).

2²¹²¹⁶ is an apocalyptic number.

21216 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21216, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (31752).

21216 is an abundant number, since it is smaller than the sum of its proper divisors (42288).

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

21216 is a wasteful number, since it uses less digits than its factorization.

21216 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 43 (or 35 counting only the distinct ones).

The product of its digits is 24, while the sum is 12.

The square root of 21216 is about 145.6571316483. The cubic root of 21216 is about 27.6835109761.

Adding to 21216 its reverse (61212), we get a palindrome (82428).

The spelling of 21216 in words is "twenty-one thousand, two hundred sixteen".