15661209 has 4 divisors (see below), whose sum is σ = 20881616. Its totient is φ = 10440804.

The previous prime is 15661193. The next prime is 15661213. The reversal of 15661209 is 90216651.

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 15661209 - 2⁴ = 15661193 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (15661279) 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, 2610199 + ... + 2610204.

It is an arithmetic number, because the mean of its divisors is an integer number (5220404).

Almost surely, 2^15661209 is an apocalyptic number.

It is an amenable number.

15661209 is a deficient number, since it is larger than the sum of its proper divisors (5220407).

15661209 is an equidigital number, since it uses as much as digits as its factorization.

15661209 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5220406.

The product of its (nonzero) digits is 3240, while the sum is 30.

The square root of 15661209 is about 3957.4245412894. The cubic root of 15661209 is about 250.1929656853.

It can be divided in two parts, 1566 and 1209, that added together give a triangular number (2775 = T₇₄).

The spelling of 15661209 in words is "fifteen million, six hundred sixty-one thousand, two hundred nine".