Yes, 191 is a prime number. Its only factors are 1 and 191.
A number is prime if no smaller prime up to its square root divides it. The square root of 191 is about 13.82, so it is enough to test the primes 2, 3, 5, 7, 11 and 13.
| Divide by | Remainder | Divides evenly? |
|---|---|---|
| 2 | 1 | No |
| 3 | 2 | No |
| 5 | 1 | No |
| 7 | 2 | No |
| 11 | 4 | No |
| 13 | 9 | No |
None of them divides 191 evenly, so 191 is prime.
| Number | Prime? |
|---|---|
| 187 | No, 11 × 17 |
| 188 | No, 2 × 2 × 47 |
| 189 | No, 3 × 3 × 3 × 7 |
| 190 | No, 2 × 5 × 19 |
| 192 | No, 2 × 2 × 2 × 2 × 2 × 2 × 3 |
| 193 | Yes |
| 194 | No, 2 × 97 |
| 195 | No, 3 × 5 × 13 |
More about 191: factors of 191 · square root of 191 · 191 in binary.
Another number? Use the prime number checker for any value, or see the list of prime numbers.