The number 21813
21813 is a composite number with 8 divisors. Its prime factorisation is 3 × 11 × 661. Divisors (8): 1, 3, 11, 33, 661, 1983, 7271, 21813.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21813: a happy number: repeatedly adding the squares of its digits reaches 1.
- 2
21813 is composite. Its prime factorisation is
- 3
The divisors add up to 31776; without 21813 itself that is 9963, so 21813 is deficient (9963 is less than 21813). Euler's totient φ(21813) = 13200 numbers up to 21813 share no factor with it.
- 4
The digits of 21813 add up to 15, and repeating that until one digit is left gives the digital root 6. The Collatz sequence (halve if even, else 3n+1) takes 69 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21813 = 3 \times 11 \times 661 \]
21813 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 31776 |
| Euler totient φ(n) | 13200 |
| Square | 475806969 |
| Cube | 10378777414797 |
| Square root | 147.692248 |
| Digit sum / digital root | 15 / 6 |
| Collatz steps to 1 | 69 |
| Scientific notation | 2.1813 × |
21813 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand eight hundred and thirteen |
| Binary (base 2) | 101010100110101 |
| Octal (base 8) | 52465 |
| Hexadecimal (base 16) | 5535 |
| Base 3 | 1002220220 |
| As time | 21813 seconds = 6 h 3 min 33 s |
Is 21813 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 (21813 ÷ 3 = 7271) | 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 3) | 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 1) | 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 6) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 3) | A number is divisible by 10 if the last digit is 0. |
| 11 | Yes (21813 ÷ 11 = 1983) | 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 21813 a prime number?
No. 21813 = 3 × 11 × 661, so it is divisible by 3.
What are the factors of 21813?
1, 3, 11, 33, 661, 1983, 7271, 21813.
What is 21813 in binary?
101010100110101 (hexadecimal 5535, octal 52465).
What is the square root of 21813?
About 147.6922.
Is 21813 divisible by 3?
Yes. Its digits add up to 15, a multiple of 3.
What is special about the number 21813?
21813 is a happy number: repeatedly adding the squares of its digits reaches 1.