21709 has 4 divisors (see below), whose sum is σ = 23004. Its totient is φ = 20416.

The previous prime is 21701. The next prime is 21713. The reversal of 21709 is 90712.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 15625 + 6084 = 125^2 + 78^2 .

It is a cyclic number.

It is not a de Polignac number, because 21709 - 2³ = 21701 is a prime.

It is an Ulam number.

It is a Duffinian number.

21709 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21701) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 622 + ... + 655.

It is an arithmetic number, because the mean of its divisors is an integer number (5751).

2²¹⁷⁰⁹ is an apocalyptic number.

It is an amenable number.

21709 is a deficient number, since it is larger than the sum of its proper divisors (1295).

21709 is a wasteful number, since it uses less digits than its factorization.

21709 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1294.

The product of its (nonzero) digits is 126, while the sum is 19.

The square root of 21709 is about 147.3397434503. The cubic root of 21709 is about 27.8963000879.

It can be divided in two parts, 2 and 1709, that added together give a triangular number (1711 = T₅₈).

The spelling of 21709 in words is "twenty-one thousand, seven hundred nine".