The number 21681
21681 is a composite number with 16 divisors. Its prime factorisation is × 11 × 73. Divisors (16): 1, 3, 9, 11, 27, 33, 73, 99, 219, 297, 657, 803, 1971, 2409, 7227, 21681.
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How to work it out
- 1
21681 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
21681 is composite. Its prime factorisation is
- 3
The divisors add up to 35520; without 21681 itself that is 13839, so 21681 is deficient (13839 is less than 21681). Euler's totient φ(21681) = 12960 numbers up to 21681 share no factor with it.
- 4
The digits of 21681 add up to 18, and repeating that until one digit is left gives the digital root 9. The Collatz sequence (halve if even, else 3n+1) takes 131 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21681 = 3^{3} \times 11 \times 73 \]21681 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 16 |
| Sum of divisors | 35520 |
| Euler totient φ(n) | 12960 |
| Square | 470065761 |
| Cube | 10191495764241 |
| Square root | 147.244694 (= 3√2409) |
| Digit sum / digital root | 18 / 9 |
| Collatz steps to 1 | 131 |
| Scientific notation | 2.1681 × |
21681 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand six hundred and eighty-one |
| Binary (base 2) | 101010010110001 |
| Octal (base 8) | 52261 |
| Hexadecimal (base 16) | 54B1 |
| Base 3 | 1002202000 |
| As time | 21681 seconds = 6 h 1 min 21 s |
Is 21681 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 (21681 ÷ 3 = 7227) | A number is divisible by 3 if the digits add up to a multiple of 3. |
| 4 | No (remainder 1) | 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 | No (remainder 2) | 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 1) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | Yes (21681 ÷ 9 = 2409) | 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 (21681 ÷ 11 = 1971) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 9) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21681 a prime number?
No. 21681 = × 11 × 73, so it is divisible by 3.
What are the factors of 21681?
1, 3, 9, 11, 27, 33, 73, 99, 219, 297, 657, 803, 1971, 2409, 7227, 21681.
What is 21681 in binary?
101010010110001 (hexadecimal 54B1, octal 52261).
What is the square root of 21681?
About 147.2447, exactly 3√2409.
Is 21681 divisible by 3?
Yes. Its digits add up to 18, a multiple of 3.
What is special about the number 21681?
It is a composite number with 16 divisors.