The number 11991
11991 is a composite number with 8 divisors. Its prime factorisation is 3 × 7 × 571. Divisors (8): 1, 3, 7, 21, 571, 1713, 3997, 11991.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 11991: a Harshad number: divisible by its digit sum 21.
- 2
11991 is composite. Its prime factorisation is
- 3
The divisors add up to 18304; without 11991 itself that is 6313, so 11991 is deficient (6313 is less than 11991). Euler's totient φ(11991) = 6840 numbers up to 11991 share no factor with it.
- 4
The digits of 11991 add up to 21, and repeating that until one digit is left gives the digital root 3. The Collatz sequence (halve if even, else 3n+1) takes 81 steps to reach 1.
LaTeX source of the formulas on this page
\[ 11991 = 3 \times 7 \times 571 \]
11991 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 18304 |
| Euler totient φ(n) | 6840 |
| Square | 143784081 |
| Cube | 1724114915271 |
| Square root | 109.503425 |
| Digit sum / digital root | 21 / 3 |
| Collatz steps to 1 | 81 |
| Scientific notation | 1.1991 × |
11991 in other forms
| Form | Value |
|---|---|
| In words | eleven thousand nine hundred and ninety-one |
| Binary (base 2) | 10111011010111 |
| Octal (base 8) | 27327 |
| Hexadecimal (base 16) | 2ED7 |
| Base 3 | 121110010 |
| As time | 11991 seconds = 3 h 19 min 51 s |
Is 11991 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 | Yes (11991 ÷ 3 = 3997) | 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 3) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | Yes (11991 ÷ 7 = 1713) | 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 3) | 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 1) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 3) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 11991 a prime number?
No. 11991 = 3 × 7 × 571, so it is divisible by 3.
What are the factors of 11991?
1, 3, 7, 21, 571, 1713, 3997, 11991.
What is 11991 in binary?
10111011010111 (hexadecimal 2ED7, octal 27327).
What is the square root of 11991?
About 109.5034.
Is 11991 divisible by 3?
Yes. Its digits add up to 21, a multiple of 3.
What is special about the number 11991?
11991 is a Harshad number: divisible by its digit sum 21.