The number 21391
21391 is the 2401st prime number. Its only divisors are 1 and 21391.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21391: the 2401st prime number.
- 2
21391 is prime, so its only factors are 1 and 21391. It is the 2401st prime; the next prime is 21397 and the one before is 21383.
- 3
The divisors add up to 21392; without 21391 itself that is 1, so 21391 is deficient (1 is less than 21391). Euler's totient φ(21391) = 21390 numbers up to 21391 share no factor with it.
- 4
The digits of 21391 add up to 16, and repeating that until one digit is left gives the digital root 7. The Collatz sequence (halve if even, else 3n+1) takes 74 steps to reach 1.
21391 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 21392 |
| Euler totient φ(n) | 21390 |
| Square | 457574881 |
| Cube | 9787984279471 |
| Square root | 146.256624 |
| Digit sum / digital root | 16 / 7 |
| Collatz steps to 1 | 74 |
| Scientific notation | 2.1391 × |
21391 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand three hundred and ninety-one |
| Binary (base 2) | 101001110001111 |
| Octal (base 8) | 51617 |
| Hexadecimal (base 16) | 538F |
| Base 3 | 1002100021 |
| As time | 21391 seconds = 5 h 56 min 31 s |
Is 21391 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 1) | A number is divisible by 3 if the digits add up to a multiple of 3. |
| 4 | No (remainder 3) | 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 1) | 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 7) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 7) | 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 7) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 7) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21391 a prime number?
Yes. 21391 is divisible only by 1 and itself.
What are the factors of 21391?
1, 21391.
What is 21391 in binary?
101001110001111 (hexadecimal 538F, octal 51617).
What is the square root of 21391?
About 146.2566.
Is 21391 divisible by 3?
No. Its digits add up to 16, which is not a multiple of 3.
What is special about the number 21391?
21391 is the 2401st prime number.