The number 191
191 is a palindromic prime: it reads the same backwards and is prime, a twin prime (191 and 193 are both prime) and the 43rd prime number. Its only divisors are 1 and 191.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 191: a palindromic prime: it reads the same backwards and is prime; a twin prime (191 and 193 are both prime); the 43rd prime number; a palindrome: it reads 191 backwards too.
- 2
191 is prime, so its only factors are 1 and 191. It is the 43rd prime; the next prime is 193 and the one before is 181.
- 3
The divisors add up to 192; without 191 itself that is 1, so 191 is deficient (1 is less than 191). Euler's totient φ(191) = 190 numbers up to 191 share no factor with it.
- 4
The digits of 191 add up to 11, and repeating that until one digit is left gives the digital root 2. The Collatz sequence (halve if even, else 3n+1) takes 44 steps to reach 1.
191 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 192 |
| Euler totient φ(n) | 190 |
| Square | 36481 |
| Cube | 6967871 |
| Square root | 13.820275 |
| Digit sum / digital root | 11 / 2 |
| Collatz steps to 1 | 44 |
| Scientific notation | 1.91 × |
191 in other forms
| Form | Value |
|---|---|
| In words | one hundred and ninety-one |
| Roman numerals | CXCI |
| Binary (base 2) | 10111111 |
| Octal (base 8) | 277 |
| Hexadecimal (base 16) | BF |
| Base 3 | 21002 |
| As time | 191 seconds = 3 min 11 s |
Is 191 divisible by 2 to 12?
| Divisor | Answer | Rule |
|---|---|---|
| 2 | No (remainder 1) | A number is divisible by 2 if the last digit is even (0, 2, 4, 6, 8). |
| 3 | No (remainder 2) | A number is divisible by 3 if the digits add up to a multiple of 3. |
| 4 | No (remainder 3) | A number is divisible by 4 if the last two digits make a multiple of 4. |
| 5 | No (remainder 1) | A number is divisible by 5 if the last digit is 0 or 5. |
| 6 | No (remainder 5) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 2) | A number is divisible by 7 if double the last digit and subtract it from the rest: the result is a multiple of 7. |
| 8 | No (remainder 7) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 2) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 1) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 4) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 11) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 191 a prime number?
Yes. 191 is divisible only by 1 and itself.
What are the factors of 191?
1, 191.
What is 191 in binary?
10111111 (hexadecimal BF, octal 277).
What is the square root of 191?
About 13.8203.
What is 191 in Roman numerals?
CXCI.
Is 191 divisible by 3?
No. Its digits add up to 11, which is not a multiple of 3.
What is special about the number 191?
191 is a palindromic prime: it reads the same backwards and is prime; a twin prime (191 and 193 are both prime); the 43rd prime number; a palindrome: it reads 191 backwards too.