The number 21491
21491 is a twin prime (21491 and 21493 are both prime) and the 2410th prime number. Its only divisors are 1 and 21491.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21491: a twin prime (21491 and 21493 are both prime); the 2410th prime number; a happy number: repeatedly adding the squares of its digits reaches 1.
- 2
21491 is prime, so its only factors are 1 and 21491. It is the 2410th prime; the next prime is 21493 and the one before is 21487.
- 3
The divisors add up to 21492; without 21491 itself that is 1, so 21491 is deficient (1 is less than 21491). Euler's totient φ(21491) = 21490 numbers up to 21491 share no factor with it.
- 4
The digits of 21491 add up to 17, and repeating that until one digit is left gives the digital root 8. The Collatz sequence (halve if even, else 3n+1) takes 56 steps to reach 1.
21491 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 21492 |
| Euler totient φ(n) | 21490 |
| Square | 461863081 |
| Cube | 9925899473771 |
| Square root | 146.598090 |
| Digit sum / digital root | 17 / 8 |
| Collatz steps to 1 | 56 |
| Scientific notation | 2.1491 × |
21491 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand four hundred and ninety-one |
| Binary (base 2) | 101001111110011 |
| Octal (base 8) | 51763 |
| Hexadecimal (base 16) | 53F3 |
| Base 3 | 1002110222 |
| As time | 21491 seconds = 5 h 58 min 11 s |
Is 21491 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 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 5) | 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 3) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 8) | 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 11) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21491 a prime number?
Yes. 21491 is divisible only by 1 and itself.
What are the factors of 21491?
1, 21491.
What is 21491 in binary?
101001111110011 (hexadecimal 53F3, octal 51763).
What is the square root of 21491?
About 146.5981.
Is 21491 divisible by 3?
No. Its digits add up to 17, which is not a multiple of 3.
What is special about the number 21491?
21491 is a twin prime (21491 and 21493 are both prime); the 2410th prime number; a happy number: repeatedly adding the squares of its digits reaches 1.