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