The number 21991
21991 is the 2463rd prime number. Its only divisors are 1 and 21991.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21991: the 2463rd prime number.
- 2
21991 is prime, so its only factors are 1 and 21991. It is the 2463rd prime; the next prime is 21997 and the one before is 21977.
- 3
The divisors add up to 21992; without 21991 itself that is 1, so 21991 is deficient (1 is less than 21991). Euler's totient φ(21991) = 21990 numbers up to 21991 share no factor with it.
- 4
The digits of 21991 add up to 22, and repeating that until one digit is left gives the digital root 4. The Collatz sequence (halve if even, else 3n+1) takes 100 steps to reach 1.
21991 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 21992 |
| Euler totient φ(n) | 21990 |
| Square | 483604081 |
| Cube | 10634937345271 |
| Square root | 148.293628 |
| Digit sum / digital root | 22 / 4 |
| Collatz steps to 1 | 100 |
| Scientific notation | 2.1991 × |
21991 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand nine hundred and ninety-one |
| Binary (base 2) | 101010111100111 |
| Octal (base 8) | 52747 |
| Hexadecimal (base 16) | 55E7 |
| Base 3 | 1010011111 |
| As time | 21991 seconds = 6 h 6 min 31 s |
Is 21991 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 4) | 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 4) | 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 2) | 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 21991 a prime number?
Yes. 21991 is divisible only by 1 and itself.
What are the factors of 21991?
1, 21991.
What is 21991 in binary?
101010111100111 (hexadecimal 55E7, octal 52747).
What is the square root of 21991?
About 148.2936.
Is 21991 divisible by 3?
No. Its digits add up to 22, which is not a multiple of 3.
What is special about the number 21991?
21991 is the 2463rd prime number.