21520 has 20 divisors (see below), whose sum is σ = 50220. Its totient is φ = 8576.

The previous prime is 21517. The next prime is 21521. The reversal of 21520 is 2512.

21520 is nontrivially palindromic in base 3.

It can be written as a sum of positive squares in 2 ways, for example, as 4096 + 17424 = 64^2 + 132^2 .

It is a tau number, because it is divible by the number of its divisors (20).

It is a super-2 number, since 2×21520² = 926220800, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (10).

It is a nialpdrome in base 16.

It is a junction number, because it is equal to n+sod(n) for n = 21497 and 21506.

It is not an unprimeable number, because it can be changed into a prime (21521) 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, 55 + ... + 214.

It is an arithmetic number, because the mean of its divisors is an integer number (2511).

2²¹⁵²⁰ is an apocalyptic number.

21520 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21520 is an abundant number, since it is smaller than the sum of its proper divisors (28700).

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

It is a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25110).

21520 is a wasteful number, since it uses less digits than its factorization.

21520 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 282 (or 276 counting only the distinct ones).

The product of its (nonzero) digits is 20, while the sum is 10.

The square root of 21520 is about 146.6969665671. The cubic root of 21520 is about 27.8151083275.

The spelling of 21520 in words is "twenty-one thousand, five hundred twenty".