21409 has 4 divisors (see below), whose sum is σ = 21760. Its totient is φ = 21060.

The previous prime is 21407. The next prime is 21419. The reversal of 21409 is 90412.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21409 - 2¹ = 21407 is a prime.

It is a Duffinian number.

21409 is a lucky number.

It is a junction number, because it is equal to n+sod(n) for n = 21392 and 21401.

It is not an unprimeable number, because it can be changed into a prime (21401) 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 3 ways as a sum of consecutive naturals, for example, 57 + ... + 214.

It is an arithmetic number, because the mean of its divisors is an integer number (5440).

2²¹⁴⁰⁹ is an apocalyptic number.

It is an amenable number.

21409 is a deficient number, since it is larger than the sum of its proper divisors (351).

21409 is an equidigital number, since it uses as much as digits as its factorization.

21409 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 350.

The product of its (nonzero) digits is 72, while the sum is 16.

The square root of 21409 is about 146.3181465164. The cubic root of 21409 is about 27.7672024929.

The spelling of 21409 in words is "twenty-one thousand, four hundred nine".