Is 1991 a prime number?
No, 1991 is not a prime number. It is divisible by 11: 1991 = 11 × 181.
Is 1991 prime?
Pick any whole number from 1 to 10,000.
How to work it out
- 1
A prime number has exactly two factors: 1 and itself.
- 2
It is enough to test the primes up to √1991 ≈ 44.62. If none of them divides 1991, it is prime.
- 3
11 divides 1991 exactly, so it is not prime. Its prime factorisation is
LaTeX source of the formulas on this page
\[ 1991 = 11 \times 181 \]
Testing 1991 by the primes up to its square root
| Prime | Division | Result |
|---|---|---|
| 2 | 1991 ÷ 2 | leaves 1 |
| 3 | 1991 ÷ 3 | leaves 2 |
| 5 | 1991 ÷ 5 | leaves 1 |
| 7 | 1991 ÷ 7 | leaves 3 |
| 11 | 1991 ÷ 11 | divides exactly (181) |
| 13 | 1991 ÷ 13 | leaves 2 |
| 17 | 1991 ÷ 17 | leaves 2 |
| 19 | 1991 ÷ 19 | leaves 15 |
| 23 | 1991 ÷ 23 | leaves 13 |
| 29 | 1991 ÷ 29 | leaves 19 |
| 31 | 1991 ÷ 31 | leaves 7 |
| 37 | 1991 ÷ 37 | leaves 30 |
Questions
Is 1991 a prime number?
No, 1991 is not a prime number. It is divisible by 11: 1991 = 11 × 181.
What is the next prime after 1991?
1993.
What is the prime number before 1991?
1987.
What are the factors of 1991?
1, 11, 181, 1991.