The number 20981
20981 is a twin prime (20981 and 20983 are both prime) and the 2359th prime number. Its only divisors are 1 and 20981.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 20981: a twin prime (20981 and 20983 are both prime); the 2359th prime number.
- 2
20981 is prime, so its only factors are 1 and 20981. It is the 2359th prime; the next prime is 20983 and the one before is 20963.
- 3
The divisors add up to 20982; without 20981 itself that is 1, so 20981 is deficient (1 is less than 20981). Euler's totient φ(20981) = 20980 numbers up to 20981 share no factor with it.
- 4
The digits of 20981 add up to 20, and repeating that until one digit is left gives the digital root 2. The Collatz sequence (halve if even, else 3n+1) takes 105 steps to reach 1.
20981 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 20982 |
| Euler totient φ(n) | 20980 |
| Square | 440202361 |
| Cube | 9235885736141 |
| Square root | 144.848196 |
| Digit sum / digital root | 20 / 2 |
| Collatz steps to 1 | 105 |
| Scientific notation | 2.0981 × |
20981 in other forms
| Form | Value |
|---|---|
| In words | twenty thousand nine hundred and eighty-one |
| Binary (base 2) | 101000111110101 |
| Octal (base 8) | 50765 |
| Hexadecimal (base 16) | 51F5 |
| Base 3 | 1001210002 |
| As time | 20981 seconds = 5 h 49 min 41 s |
Is 20981 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 2) | 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 4) | 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 20981 a prime number?
Yes. 20981 is divisible only by 1 and itself.
What are the factors of 20981?
1, 20981.
What is 20981 in binary?
101000111110101 (hexadecimal 51F5, octal 50765).
What is the square root of 20981?
About 144.8482.
Is 20981 divisible by 3?
No. Its digits add up to 20, which is not a multiple of 3.
What is special about the number 20981?
20981 is a twin prime (20981 and 20983 are both prime); the 2359th prime number.