21509 has 4 divisors (see below), whose sum is σ = 21804. Its totient is φ = 21216.

The previous prime is 21503. The next prime is 21517. The reversal of 21509 is 90512.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 9409 + 12100 = 97^2 + 110^2 .

It is a cyclic number.

It is not a de Polignac number, because 21509 - 2⁴ = 21493 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a zygodrome in base 4.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21503) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 59 + ... + 215.

It is an arithmetic number, because the mean of its divisors is an integer number (5451).

2²¹⁵⁰⁹ is an apocalyptic number.

It is an amenable number.

21509 is a deficient number, since it is larger than the sum of its proper divisors (295).

21509 is a wasteful number, since it uses less digits than its factorization.

21509 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 294.

The product of its (nonzero) digits is 90, while the sum is 17.

The square root of 21509 is about 146.6594695204. The cubic root of 21509 is about 27.8103682665.

The spelling of 21509 in words is "twenty-one thousand, five hundred nine".