The number 7991
7991 is a composite number with 4 divisors. Its prime factorisation is 61 × 131. Divisors (4): 1, 61, 131, 7991.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 7991: a semiprime: 61 × 131, the product of two primes.
- 2
7991 is composite. Its prime factorisation is
- 3
The divisors add up to 8184; without 7991 itself that is 193, so 7991 is deficient (193 is less than 7991). Euler's totient φ(7991) = 7800 numbers up to 7991 share no factor with it.
- 4
The digits of 7991 add up to 26, and repeating that until one digit is left gives the digital root 8. The Collatz sequence (halve if even, else 3n+1) takes 83 steps to reach 1.
LaTeX source of the formulas on this page
\[ 7991 = 61 \times 131 \]
7991 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 8184 |
| Euler totient φ(n) | 7800 |
| Square | 63856081 |
| Cube | 510273943271 |
| Square root | 89.392393 |
| Digit sum / digital root | 26 / 8 |
| Collatz steps to 1 | 83 |
| Scientific notation | 7.991 × |
7991 in other forms
| Form | Value |
|---|---|
| In words | seven thousand nine hundred and ninety-one |
| Binary (base 2) | 1111100110111 |
| Octal (base 8) | 17467 |
| Hexadecimal (base 16) | 1F37 |
| Base 3 | 101221222 |
| As time | 7991 seconds = 2 h 13 min 11 s |
Is 7991 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 4) | 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 8) | 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 5) | 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 7991 a prime number?
No. 7991 = 61 × 131, so it is divisible by 61.
What are the factors of 7991?
1, 61, 131, 7991.
What is 7991 in binary?
1111100110111 (hexadecimal 1F37, octal 17467).
What is the square root of 7991?
About 89.3924.
Is 7991 divisible by 3?
No. Its digits add up to 26, which is not a multiple of 3.
What is special about the number 7991?
7991 is a semiprime: 61 × 131, the product of two primes.