The number 21009
21009 is a composite number with 8 divisors. Its prime factorisation is 3 × 47 × 149. Divisors (8): 1, 3, 47, 141, 149, 447, 7003, 21009.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21009: a happy number: repeatedly adding the squares of its digits reaches 1.
- 2
21009 is composite. Its prime factorisation is
- 3
The divisors add up to 28800; without 21009 itself that is 7791, so 21009 is deficient (7791 is less than 21009). Euler's totient φ(21009) = 13616 numbers up to 21009 share no factor with it.
- 4
The digits of 21009 add up to 12, and repeating that until one digit is left gives the digital root 3. The Collatz sequence (halve if even, else 3n+1) takes 30 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21009 = 3 \times 47 \times 149 \]
21009 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 28800 |
| Euler totient φ(n) | 13616 |
| Square | 441378081 |
| Cube | 9272912103729 |
| Square root | 144.944817 |
| Digit sum / digital root | 12 / 3 |
| Collatz steps to 1 | 30 |
| Scientific notation | 2.1009 × |
21009 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand and nine |
| Binary (base 2) | 101001000010001 |
| Octal (base 8) | 51021 |
| Hexadecimal (base 16) | 5211 |
| Base 3 | 1001211010 |
| As time | 21009 seconds = 5 h 50 min 9 s |
Is 21009 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 (21009 ÷ 3 = 7003) | 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 4) | 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 | No (remainder 3) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 9) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 10) | 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 21009 a prime number?
No. 21009 = 3 × 47 × 149, so it is divisible by 3.
What are the factors of 21009?
1, 3, 47, 141, 149, 447, 7003, 21009.
What is 21009 in binary?
101001000010001 (hexadecimal 5211, octal 51021).
What is the square root of 21009?
About 144.9448.
Is 21009 divisible by 3?
Yes. Its digits add up to 12, a multiple of 3.
What is special about the number 21009?
21009 is a happy number: repeatedly adding the squares of its digits reaches 1.