The number 20991
20991 is a composite number with 4 divisors. Its prime factorisation is 3 × 6997. Divisors (4): 1, 3, 6997, 20991.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 20991: a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 6997, the product of two primes.
- 2
20991 is composite. Its prime factorisation is
- 3
The divisors add up to 27992; without 20991 itself that is 7001, so 20991 is deficient (7001 is less than 20991). Euler's totient φ(20991) = 13992 numbers up to 20991 share no factor with it.
- 4
The digits of 20991 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 87 steps to reach 1.
LaTeX source of the formulas on this page
\[ 20991 = 3 \times 6997 \]
20991 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 27992 |
| Euler totient φ(n) | 13992 |
| Square | 440622081 |
| Cube | 9249098102271 |
| Square root | 144.882711 |
| Digit sum / digital root | 21 / 3 |
| Collatz steps to 1 | 87 |
| Scientific notation | 2.0991 × |
20991 in other forms
| Form | Value |
|---|---|
| In words | twenty thousand nine hundred and ninety-one |
| Binary (base 2) | 101000111111111 |
| Octal (base 8) | 50777 |
| Hexadecimal (base 16) | 51FF |
| Base 3 | 1001210110 |
| As time | 20991 seconds = 5 h 49 min 51 s |
Is 20991 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 (20991 ÷ 3 = 6997) | 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 3) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 5) | 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 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 3) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 20991 a prime number?
No. 20991 = 3 × 6997, so it is divisible by 3.
What are the factors of 20991?
1, 3, 6997, 20991.
What is 20991 in binary?
101000111111111 (hexadecimal 51FF, octal 50777).
What is the square root of 20991?
About 144.8827.
Is 20991 divisible by 3?
Yes. Its digits add up to 21, a multiple of 3.
What is special about the number 20991?
20991 is a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 6997, the product of two primes.