The number 21987
21987 is a composite number with 12 divisors. Its prime factorisation is × 7 × 349. Divisors (12): 1, 3, 7, 9, 21, 63, 349, 1047, 2443, 3141, 7329, 21987.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
21987 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
21987 is composite. Its prime factorisation is
- 3
The divisors add up to 36400; without 21987 itself that is 14413, so 21987 is deficient (14413 is less than 21987). Euler's totient φ(21987) = 12528 numbers up to 21987 share no factor with it.
- 4
The digits of 21987 add up to 27, 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
\[ 21987 = 3^{2} \times 7 \times 349 \]21987 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 12 |
| Sum of divisors | 36400 |
| Euler totient φ(n) | 12528 |
| Square | 483428169 |
| Cube | 10629135151803 |
| Square root | 148.280140 (= 3√2443) |
| Digit sum / digital root | 27 / 9 |
| Collatz steps to 1 | 131 |
| Scientific notation | 2.1987 × |
21987 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand nine hundred and eighty-seven |
| Binary (base 2) | 101010111100011 |
| Octal (base 8) | 52743 |
| Hexadecimal (base 16) | 55E3 |
| Base 3 | 1010011100 |
| As time | 21987 seconds = 6 h 6 min 27 s |
Is 21987 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 (21987 ÷ 3 = 7329) | 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 2) | 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 (21987 ÷ 7 = 3141) | 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 3) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | Yes (21987 ÷ 9 = 2443) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 7) | 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 3) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21987 a prime number?
No. 21987 = × 7 × 349, so it is divisible by 3.
What are the factors of 21987?
1, 3, 7, 9, 21, 63, 349, 1047, 2443, 3141, 7329, 21987.
What is 21987 in binary?
101010111100011 (hexadecimal 55E3, octal 52743).
What is the square root of 21987?
About 148.2801, exactly 3√2443.
Is 21987 divisible by 3?
Yes. Its digits add up to 27, a multiple of 3.
What is special about the number 21987?
It is a composite number with 12 divisors.