21609 has 15 divisors (see below), whose sum is σ = 36413. Its totient is φ = 12348.

The previous prime is 21601. The next prime is 21611. The reversal of 21609 is 90612.

21609 = T₁₄₆ + T₁₄₇.

The square root of 21609 is 147.

It is a perfect power (a square), and thus also a powerful number.

It is not a de Polignac number, because 21609 - 2³ = 21601 is a prime.

It is a Duffinian number.

21609 is a lucky number.

It is a junction number, because it is equal to n+sod(n) for n = 21591 and 21600.

It is not an unprimeable number, because it can be changed into a prime (21601) 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 14 ways as a sum of consecutive naturals, for example, 3084 + ... + 3090.

2²¹⁶⁰⁹ is an apocalyptic number.

21609 is the 147-th square number.

21609 is the 74-th centered octagonal number.

It is an amenable number.

21609 is a deficient number, since it is larger than the sum of its proper divisors (14804).

21609 is an frugal number, since it uses more digits than its factorization.

21609 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 34 (or 10 counting only the distinct ones).

The product of its (nonzero) digits is 108, while the sum is 18.

The cubic root of 21609 is about 27.8534004553.

It can be divided in two parts, 21 and 609, that added together give a triangular number (630 = T₃₅).

The spelling of 21609 in words is "twenty-one thousand, six hundred nine".