21009409 has 4 divisors (see below), whose sum is σ = 21126960. Its totient is φ = 20891860.

The previous prime is 21009407. The next prime is 21009449. The reversal of 21009409 is 90490012.

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 21009409 - 2¹ = 21009407 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21009403) 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, 58507 + ... + 58864.

It is an arithmetic number, because the mean of its divisors is an integer number (5281740).

Almost surely, 2^21009409 is an apocalyptic number.

It is an amenable number.

21009409 is a deficient number, since it is larger than the sum of its proper divisors (117551).

21009409 is a wasteful number, since it uses less digits than its factorization.

21009409 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 117550.

The product of its (nonzero) digits is 648, while the sum is 25.

The square root of 21009409 is about 4583.6021860541. The cubic root of 21009409 is about 275.9336157994.

The spelling of 21009409 in words is "twenty-one million, nine thousand, four hundred nine".