1991 has 4 divisors (see below), whose sum is σ = 2184. Its totient is φ = 1800.

The previous prime is 1987. The next prime is 1993.

1991 is nontrivially palindromic in base 10 and base 16.

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

It is a cyclic number.

It is not a de Polignac number, because 1991 - 2² = 1987 is a prime.

It is a Duffinian number.

1991 is an undulating number in base 16.

It is a plaindrome in base 12.

It is a nialpdrome in base 7 and base 13.

It is a zygodrome in base 2.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1993) 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, 80 + ... + 101.

It is an arithmetic number, because the mean of its divisors is an integer number (546).

1991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

1991 is a deficient number, since it is larger than the sum of its proper divisors (193).

1991 is a wasteful number, since it uses less digits than its factorization.

1991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 192.

The product of its digits is 81, while the sum is 20.

The square root of 1991 is about 44.6206230346. The cubic root of 1991 is about 12.5802832639.

The spelling of 1991 in words is "one thousand, nine hundred ninety-one".