1991991 has 4 divisors (see below), whose sum is σ = 2655992. Its totient is φ = 1327992.

The previous prime is 1991989. The next prime is 1991993.

1991991 is nontrivially palindromic in base 10.

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

It is an interprime number because it is at equal distance from previous prime (1991989) and next prime (1991993).

It is not a de Polignac number, because 1991991 - 2¹ = 1991989 is a prime.

It is a D-number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991993) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 331996 + ... + 332001.

It is an arithmetic number, because the mean of its divisors is an integer number (663998).

2^1991991 is an apocalyptic number.

1991991 is a deficient number, since it is larger than the sum of its proper divisors (664001).

1991991 is an equidigital number, since it uses as much as digits as its factorization.

1991991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 664000.

The product of its digits is 6561, while the sum is 39.

The square root of 1991991 is about 1411.3791127830. The cubic root of 1991991 is about 125.8237015370.

The spelling of 1991991 in words is "one million, nine hundred ninety-one thousand, nine hundred ninety-one".