21720 has 32 divisors (see below), whose sum is σ = 65520. Its totient is φ = 5760.

The previous prime is 21713. The next prime is 21727. The reversal of 21720 is 2712.

It is an interprime number because it is at equal distance from previous prime (21713) and next prime (21727).

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

It is a junction number, because it is equal to n+sod(n) for n = 21696 and 21705.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21727) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 30 + ... + 210.

2²¹⁷²⁰ is an apocalyptic number.

21720 is a gapful number since it is divisible by the number (20) 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 21720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32760).

21720 is an abundant number, since it is smaller than the sum of its proper divisors (43800).

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

21720 is a wasteful number, since it uses less digits than its factorization.

21720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 195 (or 191 counting only the distinct ones).

The product of its (nonzero) digits is 28, while the sum is 12.

The square root of 21720 is about 147.3770674155. The cubic root of 21720 is about 27.9010109987.

It can be divided in two parts, 21 and 720, that added together give a triangular number (741 = T₃₈).

The spelling of 21720 in words is "twenty-one thousand, seven hundred twenty", and thus it is an iban number.