209991 has 4 divisors (see below), whose sum is σ = 279992. Its totient is φ = 139992.

The previous prime is 209987. The next prime is 210011. The reversal of 209991 is 199902.

209991 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a semiprime because it is the product of two primes.

It is not a de Polignac number, because 209991 - 2² = 209987 is a prime.

It is an Ulam number.

It is a D-number.

It is a Duffinian number.

It is a plaindrome in base 16.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (209971) 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, 34996 + ... + 35001.

It is an arithmetic number, because the mean of its divisors is an integer number (69998).

2²⁰⁹⁹⁹¹ is an apocalyptic number.

209991 is a deficient number, since it is larger than the sum of its proper divisors (70001).

209991 is an equidigital number, since it uses as much as digits as its factorization.

209991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 70000.

The product of its (nonzero) digits is 1458, while the sum is 30.

The square root of 209991 is about 458.2477495853. The cubic root of 209991 is about 59.4383703838.

It can be divided in two parts, 20 and 9991, that added together give a triangular number (10011 = T₁₄₁).

The spelling of 209991 in words is "two hundred nine thousand, nine hundred ninety-one".