2181991 has 4 divisors (see below), whose sum is σ = 2493712. Its totient is φ = 1870272.

The previous prime is 2181973. The next prime is 2182007. The reversal of 2181991 is 1991812.

2181991 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 a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-2181991 is a prime.

It is a super-2 number, since 2×2181991² = 9522169448162, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (2182991) 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, 155850 + ... + 155863.

It is an arithmetic number, because the mean of its divisors is an integer number (623428).

2^2181991 is an apocalyptic number.

2181991 is a deficient number, since it is larger than the sum of its proper divisors (311721).

2181991 is an equidigital number, since it uses as much as digits as its factorization.

2181991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 311720.

The product of its digits is 1296, while the sum is 31.

The square root of 2181991 is about 1477.1563898247. The cubic root of 2181991 is about 129.7032877959.

It can be divided in two parts, 218 and 1991, that added together give a square (2209 = 47²).

The spelling of 2181991 in words is "two million, one hundred eighty-one thousand, nine hundred ninety-one".