The number 21405
21405 is a composite number with 8 divisors. Its prime factorisation is 3 × 5 × 1427. Divisors (8): 1, 3, 5, 15, 1427, 4281, 7135, 21405.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
21405 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
21405 is composite. Its prime factorisation is
- 3
The divisors add up to 34272; without 21405 itself that is 12867, so 21405 is deficient (12867 is less than 21405). Euler's totient φ(21405) = 11408 numbers up to 21405 share no factor with it.
- 4
The digits of 21405 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 193 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21405 = 3 \times 5 \times 1427 \]
21405 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 34272 |
| Euler totient φ(n) | 11408 |
| Square | 458174025 |
| Cube | 9807215005125 |
| Square root | 146.304477 |
| Digit sum / digital root | 12 / 3 |
| Collatz steps to 1 | 193 |
| Scientific notation | 2.1405 × |
21405 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand four hundred and five |
| Binary (base 2) | 101001110011101 |
| Octal (base 8) | 51635 |
| Hexadecimal (base 16) | 539D |
| Base 3 | 1002100210 |
| As time | 21405 seconds = 5 h 56 min 45 s |
Is 21405 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 (21405 ÷ 3 = 7135) | 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 (21405 ÷ 5 = 4281) | 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 6) | 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 3) | 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 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 21405 a prime number?
No. 21405 = 3 × 5 × 1427, so it is divisible by 3.
What are the factors of 21405?
1, 3, 5, 15, 1427, 4281, 7135, 21405.
What is 21405 in binary?
101001110011101 (hexadecimal 539D, octal 51635).
What is the square root of 21405?
About 146.3045.
Is 21405 divisible by 3?
Yes. Its digits add up to 12, a multiple of 3.
What is special about the number 21405?
It is a composite number with 8 divisors.