The number 21808
21808 is a composite number with 20 divisors. Its prime factorisation is × 29 × 47. Divisors (20): 1, 2, 4, 8, 16, 29, 47, 58, 94, 116, 188, 232, 376, 464, 752, 1363 … 21808.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21808: a happy number: repeatedly adding the squares of its digits reaches 1.
- 2
21808 is composite. Its prime factorisation is
- 3
The divisors add up to 44640; without 21808 itself that is 22832, so 21808 is abundant (22832 is more than 21808). Euler's totient φ(21808) = 10304 numbers up to 21808 share no factor with it.
- 4
The digits of 21808 add up to 19, and repeating that until one digit is left gives the digital root 1. The Collatz sequence (halve if even, else 3n+1) takes 69 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21808 = 2^{4} \times 29 \times 47 \]21808 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 20 |
| Sum of divisors | 44640 |
| Euler totient φ(n) | 10304 |
| Square | 475588864 |
| Cube | 10371641946112 |
| Square root | 147.675320 (= 4√1363) |
| Digit sum / digital root | 19 / 1 |
| Collatz steps to 1 | 69 |
| Scientific notation | 2.1808 × |
21808 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand eight hundred and eight |
| Binary (base 2) | 101010100110000 |
| Octal (base 8) | 52460 |
| Hexadecimal (base 16) | 5530 |
| Base 3 | 1002220201 |
| As time | 21808 seconds = 6 h 3 min 28 s |
Is 21808 divisible by 2 to 12?
| Divisor | Answer | Rule |
|---|---|---|
| 2 | Yes (21808 ÷ 2 = 10904) | A number is divisible by 2 if the last digit is even (0, 2, 4, 6, 8). |
| 3 | No (remainder 1) | A number is divisible by 3 if the digits add up to a multiple of 3. |
| 4 | Yes (21808 ÷ 4 = 5452) | A number is divisible by 4 if the last two digits make a multiple of 4. |
| 5 | No (remainder 3) | A number is divisible by 5 if the last digit is 0 or 5. |
| 6 | No (remainder 4) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 3) | 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 | Yes (21808 ÷ 8 = 2726) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 1) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 8) | 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 4) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21808 a prime number?
No. 21808 = × 29 × 47, so it is divisible by 2.
What are the factors of 21808?
1, 2, 4, 8, 16, 29, 47, 58, 94, 116, 188, 232, 376, 464, 752, 1363, 2726, 5452, 10904, 21808.
What is 21808 in binary?
101010100110000 (hexadecimal 5530, octal 52460).
What is the square root of 21808?
About 147.6753, exactly 4√1363.
Is 21808 divisible by 3?
No. Its digits add up to 19, which is not a multiple of 3.
What is special about the number 21808?
21808 is a happy number: repeatedly adding the squares of its digits reaches 1.