The number 21509
21509 is a composite number with 4 divisors. Its prime factorisation is 137 × 157. Divisors (4): 1, 137, 157, 21509.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21509: a semiprime: 137 × 157, the product of two primes.
- 2
21509 is composite. Its prime factorisation is
- 3
The divisors add up to 21804; without 21509 itself that is 295, so 21509 is deficient (295 is less than 21509). Euler's totient φ(21509) = 21216 numbers up to 21509 share no factor with it.
- 4
The digits of 21509 add up to 17, and repeating that until one digit is left gives the digital root 8. The Collatz sequence (halve if even, else 3n+1) takes 74 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21509 = 137 \times 157 \]
21509 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 21804 |
| Euler totient φ(n) | 21216 |
| Square | 462637081 |
| Cube | 9950860975229 |
| Square root | 146.659470 |
| Digit sum / digital root | 17 / 8 |
| Collatz steps to 1 | 74 |
| Scientific notation | 2.1509 × |
21509 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand five hundred and nine |
| Binary (base 2) | 101010000000101 |
| Octal (base 8) | 52005 |
| Hexadecimal (base 16) | 5405 |
| Base 3 | 1002111122 |
| As time | 21509 seconds = 5 h 58 min 29 s |
Is 21509 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 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 5) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 5) | 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 5) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 8) | 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 4) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 5) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21509 a prime number?
No. 21509 = 137 × 157, so it is divisible by 137.
What are the factors of 21509?
1, 137, 157, 21509.
What is 21509 in binary?
101010000000101 (hexadecimal 5405, octal 52005).
What is the square root of 21509?
About 146.6595.
Is 21509 divisible by 3?
No. Its digits add up to 17, which is not a multiple of 3.
What is special about the number 21509?
21509 is a semiprime: 137 × 157, the product of two primes.