The number 21791
21791 is a composite number with 8 divisors. Its prime factorisation is 7 × 11 × 283. Divisors (8): 1, 7, 11, 77, 283, 1981, 3113, 21791.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
21791 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
21791 is composite. Its prime factorisation is
- 3
The divisors add up to 27264; without 21791 itself that is 5473, so 21791 is deficient (5473 is less than 21791). Euler's totient φ(21791) = 16920 numbers up to 21791 share no factor with it.
- 4
The digits of 21791 add up to 20, and repeating that until one digit is left gives the digital root 2. The Collatz sequence (halve if even, else 3n+1) takes 69 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21791 = 7 \times 11 \times 283 \]
21791 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 27264 |
| Euler totient φ(n) | 16920 |
| Square | 474847681 |
| Cube | 10347405816671 |
| Square root | 147.617750 |
| Digit sum / digital root | 20 / 2 |
| Collatz steps to 1 | 69 |
| Scientific notation | 2.1791 × |
21791 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand seven hundred and ninety-one |
| Binary (base 2) | 101010100011111 |
| Octal (base 8) | 52437 |
| Hexadecimal (base 16) | 551F |
| Base 3 | 1002220002 |
| As time | 21791 seconds = 6 h 3 min 11 s |
Is 21791 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 | Yes (21791 ÷ 7 = 3113) | 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 | Yes (21791 ÷ 11 = 1981) | 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 21791 a prime number?
No. 21791 = 7 × 11 × 283, so it is divisible by 7.
What are the factors of 21791?
1, 7, 11, 77, 283, 1981, 3113, 21791.
What is 21791 in binary?
101010100011111 (hexadecimal 551F, octal 52437).
What is the square root of 21791?
About 147.6177.
Is 21791 divisible by 3?
No. Its digits add up to 20, which is not a multiple of 3.
What is special about the number 21791?
It is a composite number with 8 divisors.