1991209 has 4 divisors (see below), whose sum is σ = 2172240. Its totient is φ = 1810180.

The previous prime is 1991189. The next prime is 1991239. The reversal of 1991209 is 9021991.

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 1991209 - 2⁵ = 1991177 is a prime.

It is an Ulam number.

It is a Duffinian number.

1991209 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (1991239) 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, 90499 + ... + 90520.

It is an arithmetic number, because the mean of its divisors is an integer number (543060).

2^1991209 is an apocalyptic number.

It is an amenable number.

1991209 is a deficient number, since it is larger than the sum of its proper divisors (181031).

1991209 is a wasteful number, since it uses less digits than its factorization.

1991209 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 181030.

The product of its (nonzero) digits is 1458, while the sum is 31.

The square root of 1991209 is about 1411.1020515895. The cubic root of 1991209 is about 125.8072344256.

The spelling of 1991209 in words is "one million, nine hundred ninety-one thousand, two hundred nine".