The number 21981
21981 is a composite number with 8 divisors. Its prime factorisation is 3 × 17 × 431. Divisors (8): 1, 3, 17, 51, 431, 1293, 7327, 21981.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
21981 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
21981 is composite. Its prime factorisation is
- 3
The divisors add up to 31104; without 21981 itself that is 9123, so 21981 is deficient (9123 is less than 21981). Euler's totient φ(21981) = 13760 numbers up to 21981 share no factor with it.
- 4
The digits of 21981 add up to 21, and repeating that until one digit is left gives the digital root 3. The Collatz sequence (halve if even, else 3n+1) takes 69 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21981 = 3 \times 17 \times 431 \]
21981 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 31104 |
| Euler totient φ(n) | 13760 |
| Square | 483164361 |
| Cube | 10620435819141 |
| Square root | 148.259907 |
| Digit sum / digital root | 21 / 3 |
| Collatz steps to 1 | 69 |
| Scientific notation | 2.1981 × |
21981 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand nine hundred and eighty-one |
| Binary (base 2) | 101010111011101 |
| Octal (base 8) | 52735 |
| Hexadecimal (base 16) | 55DD |
| Base 3 | 1010011010 |
| As time | 21981 seconds = 6 h 6 min 21 s |
Is 21981 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 | Yes (21981 ÷ 3 = 7327) | 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 3) | 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 5) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 3) | 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 3) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 9) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21981 a prime number?
No. 21981 = 3 × 17 × 431, so it is divisible by 3.
What are the factors of 21981?
1, 3, 17, 51, 431, 1293, 7327, 21981.
What is 21981 in binary?
101010111011101 (hexadecimal 55DD, octal 52735).
What is the square root of 21981?
About 148.2599.
Is 21981 divisible by 3?
Yes. Its digits add up to 21, a multiple of 3.
What is special about the number 21981?
It is a composite number with 8 divisors.