21761209 has 4 divisors (see below), whose sum is σ = 21882960. Its totient is φ = 21639460.

The previous prime is 21761191. The next prime is 21761219. The reversal of 21761209 is 90216712.

It is a happy number.

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 21761209 - 2¹⁹ = 21236921 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21761219) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 60607 + ... + 60964.

It is an arithmetic number, because the mean of its divisors is an integer number (5470740).

Almost surely, 2^21761209 is an apocalyptic number.

It is an amenable number.

21761209 is a deficient number, since it is larger than the sum of its proper divisors (121751).

21761209 is a wasteful number, since it uses less digits than its factorization.

21761209 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 121750.

The product of its (nonzero) digits is 1512, while the sum is 28.

The square root of 21761209 is about 4664.8911026947. The cubic root of 21761209 is about 279.1864522795.

The spelling of 21761209 in words is "twenty-one million, seven hundred sixty-one thousand, two hundred nine".