21945 has 32 divisors (see below), whose sum is σ = 46080. Its totient is φ = 8640.

The previous prime is 21943. The next prime is 21961. The reversal of 21945 is 54912.

21945 is nontrivially palindromic in base 14.

21945 is a nontrivial binomial coefficient, being equal to C(210, 2).

It is not a de Polignac number, because 21945 - 2¹ = 21943 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (21).

Its product of digits (360) is a multiple of the sum of its prime divisors (45).

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21943) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 1146 + ... + 1164.

It is an arithmetic number, because the mean of its divisors is an integer number (1440).

2²¹⁹⁴⁵ is an apocalyptic number.

21945 is the 209-th triangular number and also the 105-th hexagonal number.

It is an amenable number.

21945 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.

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 (23040).

21945 is a wasteful number, since it uses less digits than its factorization.

21945 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 45.

The product of its digits is 360, while the sum is 21.

The square root of 21945 is about 148.1384487566. The cubic root of 21945 is about 27.9970234931.

The spelling of 21945 in words is "twenty-one thousand, nine hundred forty-five".