21798 has 24 divisors (see below), whose sum is σ = 54288. Its totient is φ = 6192.

The previous prime is 21787. The next prime is 21799. The reversal of 21798 is 89712.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21799) 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 11 ways as a sum of consecutive naturals, for example, 40 + ... + 212.

It is an arithmetic number, because the mean of its divisors is an integer number (2262).

2²¹⁷⁹⁸ is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21798, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27144).

21798 is an abundant number, since it is smaller than the sum of its proper divisors (32490).

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

21798 is a wasteful number, since it uses less digits than its factorization.

21798 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 188 (or 185 counting only the distinct ones).

The product of its digits is 1008, while the sum is 27.

The square root of 21798 is about 147.6414575924. The cubic root of 21798 is about 27.9343700981.

It can be divided in two parts, 2179 and 8, that added together give a 7-th power (2187 = 3⁷).

The spelling of 21798 in words is "twenty-one thousand, seven hundred ninety-eight".