21724 has 6 divisors (see below), whose sum is σ = 38024. Its totient is φ = 10860.

The previous prime is 21713. The next prime is 21727. The reversal of 21724 is 42712.

It is a junction number, because it is equal to n+sod(n) for n = 21698 and 21707.

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 polite number, since it can be written as a sum of consecutive naturals, namely, 2712 + ... + 2719.

2²¹⁷²⁴ is an apocalyptic number.

It is an amenable number.

21724 is a deficient number, since it is larger than the sum of its proper divisors (16300).

21724 is a wasteful number, since it uses less digits than its factorization.

21724 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5435 (or 5433 counting only the distinct ones).

The product of its digits is 112, while the sum is 16.

The square root of 21724 is about 147.3906374231. The cubic root of 21724 is about 27.9027236628.

Subtracting from 21724 its product of digits (112), we obtain a palindrome (21612).

The spelling of 21724 in words is "twenty-one thousand, seven hundred twenty-four", and thus it is an iban number.