The number 31991
31991 is the 3432nd prime number. Its only divisors are 1 and 31991.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 31991: the 3432nd prime number.
- 2
31991 is prime, so its only factors are 1 and 31991. It is the 3432nd prime; the next prime is 32003 and the one before is 31981.
- 3
The divisors add up to 31992; without 31991 itself that is 1, so 31991 is deficient (1 is less than 31991). Euler's totient φ(31991) = 31990 numbers up to 31991 share no factor with it.
- 4
The digits of 31991 add up to 23, and repeating that until one digit is left gives the digital root 5. The Collatz sequence (halve if even, else 3n+1) takes 54 steps to reach 1.
31991 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 31992 |
| Euler totient φ(n) | 31990 |
| Square | 1023424081 |
| Cube | 32740359775271 |
| Square root | 178.860281 |
| Digit sum / digital root | 23 / 5 |
| Collatz steps to 1 | 54 |
| Scientific notation | 3.1991 × |
31991 in other forms
| Form | Value |
|---|---|
| In words | thirty-one thousand nine hundred and ninety-one |
| Binary (base 2) | 111110011110111 |
| Octal (base 8) | 76367 |
| Hexadecimal (base 16) | 7CF7 |
| Base 3 | 1121212212 |
| As time | 31991 seconds = 8 h 53 min 11 s |
Is 31991 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 1) | 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 5) | 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 3) | 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 31991 a prime number?
Yes. 31991 is divisible only by 1 and itself.
What are the factors of 31991?
1, 31991.
What is 31991 in binary?
111110011110111 (hexadecimal 7CF7, octal 76367).
What is the square root of 31991?
About 178.8603.
Is 31991 divisible by 3?
No. Its digits add up to 23, which is not a multiple of 3.
What is special about the number 31991?
31991 is the 3432nd prime number.