The number 21821
21821 is the 2448th prime number. Its only divisors are 1 and 21821.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21821: the 2448th prime number.
- 2
21821 is prime, so its only factors are 1 and 21821. It is the 2448th prime; the next prime is 21839 and the one before is 21817.
- 3
The divisors add up to 21822; without 21821 itself that is 1, so 21821 is deficient (1 is less than 21821). Euler's totient φ(21821) = 21820 numbers up to 21821 share no factor with it.
- 4
The digits of 21821 add up to 14, and repeating that until one digit is left gives the digital root 5. The Collatz sequence (halve if even, else 3n+1) takes 131 steps to reach 1.
21821 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 21822 |
| Euler totient φ(n) | 21820 |
| Square | 476156041 |
| Cube | 10390200970661 |
| Square root | 147.719328 |
| Digit sum / digital root | 14 / 5 |
| Collatz steps to 1 | 131 |
| Scientific notation | 2.1821 × |
21821 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand eight hundred and twenty-one |
| Binary (base 2) | 101010100111101 |
| Octal (base 8) | 52475 |
| Hexadecimal (base 16) | 553D |
| Base 3 | 1002221012 |
| As time | 21821 seconds = 6 h 3 min 41 s |
Is 21821 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 1) | 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 2) | 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 5) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 1) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 8) | 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 21821 a prime number?
Yes. 21821 is divisible only by 1 and itself.
What are the factors of 21821?
1, 21821.
What is 21821 in binary?
101010100111101 (hexadecimal 553D, octal 52475).
What is the square root of 21821?
About 147.7193.
Is 21821 divisible by 3?
No. Its digits add up to 14, which is not a multiple of 3.
What is special about the number 21821?
21821 is the 2448th prime number.