Names of 1991
| Nominal | 1991 |
| Cardinal | one thousand nine hundred ninety-one |
| Ordinal | 1,991st |
| This number as US currency | one thousand nine hundred ninety-one dollars |
| Is 1991 an odd number? | 1991 is an odd number |
| Is 1991 an even number? | 1991 is NOT an even number |
| Is 1991 a palindrome? | 1991 is a palindrome number |
| Is 1991 a triangle number? | 1991 is NOT a triangle number |
1991 in other Bases
| Base 2 (Binary) | 111110001112 |
| Base 3 (Ternary) | 22012023 |
| Base 4 (Quaternary) | 1330134 |
| Base 5 (Quinary) | 304315 |
| Base 6 (Senary) | 131156 |
| Base 8 (Octal) | 37078 |
| Base 12 (Duodecimal) | 119B12 |
| Base 16 (Hexadecimal) | 7C716 |
| Base 20 (Vigesimal) | 4JB20 |
Primes and Factors tests for 1991
| Is 1991 a Prime Number? | 1991 is NOT a prime |
| Is 1991 a perfect number? | 1991 is NOT a perfect number |
| Is 1991 a Mersenne Prime? | 1991 is NOT a Mersenne prime |
| Is 1991 a Fermat Prime? | 1991 is NOT a Fermat prime |
| Is 1991 a Fibonacci Prime? | 1991 is NOT a Fibonacci prime |
| Is 1991 a Partition Prime? | 1991 is NOT a Partition prime |
| Is 1991 a Pell Prime? | 1991 is NOT a Pell prime |
| Prime Factors | Prime factors of 1991 |
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1991 in other Numerals
| 1991 in Roman Numeral | MCMXCI |
| 1991 in Chinese Numeral | 壹仟玖佰玖拾壹 |
| 1991 in Egyptian Numeral | 𓆼𓍪𓎎𓏺 |
| 1991 in Babylonian Numeral |   |
Roots and stuff for 1991
| Is 1991 a square number? | 1991 is NOT a square number |
| Is 1991 a cube number? | 1991 is NOT a cube number |
| Square root of the number 1991 | 44.620623034646 |
| Cube root of the number 1991 | 12.580283263893 |
Recreational math for 1991
| Number 1991 reversed | 1991 |
| Sum of the digits | 20 |
| No of digits | 4 |