21491409 has 4 divisors (see below), whose sum is σ = 28655216. Its totient is φ = 14327604.

The previous prime is 21491381. The next prime is 21491411. The reversal of 21491409 is 90419412.

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 not a de Polignac number, because 21491409 - 2⁹ = 21490897 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21491429) 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, 3581899 + ... + 3581904.

It is an arithmetic number, because the mean of its divisors is an integer number (7163804).

Almost surely, 2^21491409 is an apocalyptic number.

It is an amenable number.

21491409 is a deficient number, since it is larger than the sum of its proper divisors (7163807).

21491409 is an equidigital number, since it uses as much as digits as its factorization.

21491409 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7163806.

The product of its (nonzero) digits is 2592, while the sum is 30.

The square root of 21491409 is about 4635.8827638326. The cubic root of 21491409 is about 278.0278468732.

The spelling of 21491409 in words is "twenty-one million, four hundred ninety-one thousand, four hundred nine".