The number 3991
3991 is a composite number with 4 divisors. Its prime factorisation is 13 × 307. Divisors (4): 1, 13, 307, 3991.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 3991: a semiprime: 13 × 307, the product of two primes.
- 2
3991 is composite. Its prime factorisation is
- 3
The divisors add up to 4312; without 3991 itself that is 321, so 3991 is deficient (321 is less than 3991). Euler's totient φ(3991) = 3672 numbers up to 3991 share no factor with it.
- 4
The digits of 3991 add up to 22, and repeating that until one digit is left gives the digital root 4. The Collatz sequence (halve if even, else 3n+1) takes 51 steps to reach 1.
LaTeX source of the formulas on this page
\[ 3991 = 13 \times 307 \]
3991 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 4312 |
| Euler totient φ(n) | 3672 |
| Square | 15928081 |
| Cube | 63568971271 |
| Square root | 63.174362 |
| Digit sum / digital root | 22 / 4 |
| Collatz steps to 1 | 51 |
| Scientific notation | 3.991 × |
3991 in other forms
| Form | Value |
|---|---|
| In words | three thousand nine hundred and ninety-one |
| Roman numerals | MMMCMXCI |
| Binary (base 2) | 111110010111 |
| Octal (base 8) | 7627 |
| Hexadecimal (base 16) | F97 |
| Base 3 | 12110211 |
| As time | 3991 seconds = 1 h 6 min 31 s |
Is 3991 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 1) | 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 1) | 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 4) | 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 9) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 7) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 3991 a prime number?
No. 3991 = 13 × 307, so it is divisible by 13.
What are the factors of 3991?
1, 13, 307, 3991.
What is 3991 in binary?
111110010111 (hexadecimal F97, octal 7627).
What is the square root of 3991?
About 63.1744.
What is 3991 in Roman numerals?
MMMCMXCI.
Is 3991 divisible by 3?
No. Its digits add up to 22, which is not a multiple of 3.
What is special about the number 3991?
3991 is a semiprime: 13 × 307, the product of two primes.