The number 21005
21005 is a composite number with 4 divisors. Its prime factorisation is 5 × 4201. Divisors (4): 1, 5, 4201, 21005.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21005: a semiprime: 5 × 4201, the product of two primes.
- 2
21005 is composite. Its prime factorisation is
- 3
The divisors add up to 25212; without 21005 itself that is 4207, so 21005 is deficient (4207 is less than 21005). Euler's totient φ(21005) = 16800 numbers up to 21005 share no factor with it.
- 4
The digits of 21005 add up to 8, and repeating that until one digit is left gives the digital root 8. The Collatz sequence (halve if even, else 3n+1) takes 30 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21005 = 5 \times 4201 \]
21005 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 25212 |
| Euler totient φ(n) | 16800 |
| Square | 441210025 |
| Cube | 9267616575125 |
| Square root | 144.931018 |
| Digit sum / digital root | 8 / 8 |
| Collatz steps to 1 | 30 |
| Scientific notation | 2.1005 × |
21005 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand and five |
| Binary (base 2) | 101001000001101 |
| Octal (base 8) | 51015 |
| Hexadecimal (base 16) | 520D |
| Base 3 | 1001210222 |
| As time | 21005 seconds = 5 h 50 min 5 s |
Is 21005 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 | Yes (21005 ÷ 5 = 4201) | 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 5) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 6) | 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 21005 a prime number?
No. 21005 = 5 × 4201, so it is divisible by 5.
What are the factors of 21005?
1, 5, 4201, 21005.
What is 21005 in binary?
101001000001101 (hexadecimal 520D, octal 51015).
What is the square root of 21005?
About 144.9310.
Is 21005 divisible by 3?
No. Its digits add up to 8, which is not a multiple of 3.
What is special about the number 21005?
21005 is a semiprime: 5 × 4201, the product of two primes.