21768 has 16 divisors (see below), whose sum is σ = 54480. Its totient is φ = 7248.

The previous prime is 21767. The next prime is 21773. The reversal of 21768 is 86712.

It is a Harshad number since it is a multiple of its sum of digits (24), and also a Moran number because the ratio is a prime number: 907 = 21768 / (2 + 1 + 7 + 6 + 8).

It is not an unprimeable number, because it can be changed into a prime (21767) by changing a digit.

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 3 ways as a sum of consecutive naturals, for example, 430 + ... + 477.

It is an arithmetic number, because the mean of its divisors is an integer number (3405).

2²¹⁷⁶⁸ is an apocalyptic number.

It is an amenable number.

21768 is an abundant number, since it is smaller than the sum of its proper divisors (32712).

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 (27240).

21768 is a wasteful number, since it uses less digits than its factorization.

21768 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 916 (or 912 counting only the distinct ones).

The product of its digits is 672, while the sum is 24.

The square root of 21768 is about 147.5398251321. The cubic root of 21768 is about 27.9215491077.

It can be divided in two parts, 2 and 1768, that added together give a triangular number (1770 = T₅₉).

The spelling of 21768 in words is "twenty-one thousand, seven hundred sixty-eight".