21516 has 24 divisors (see below), whose sum is σ = 55104. Its totient is φ = 6480.

The previous prime is 21503. The next prime is 21517. The reversal of 21516 is 61512.

Added to its reverse (61512) it gives a triangular number (83028 = T₄₀₇).

It is a junction number, because it is equal to n+sod(n) for n = 21495 and 21504.

It is not an unprimeable number, because it can be changed into a prime (21517) by changing a digit.

21516 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 51 + ... + 213.

It is an arithmetic number, because the mean of its divisors is an integer number (2296).

2²¹⁵¹⁶ 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 21516, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27552).

21516 is an abundant number, since it is smaller than the sum of its proper divisors (33588).

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

21516 is a wasteful number, since it uses less digits than its factorization.

21516 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 181 (or 179 counting only the distinct ones).

The product of its digits is 60, while the sum is 15.

The square root of 21516 is about 146.6833323865. The cubic root of 21516 is about 27.8133848559.

Adding to 21516 its reverse (61512), we get a triangular number (83028 = T₄₀₇).

It can be divided in two parts, 215 and 16, that added together give a triangular number (231 = T₂₁).

The spelling of 21516 in words is "twenty-one thousand, five hundred sixteen".