The number 18991
18991 is a composite number with 4 divisors. Its prime factorisation is 7 × 2713. Divisors (4): 1, 7, 2713, 18991.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 18991: a semiprime: 7 × 2713, the product of two primes.
- 2
18991 is composite. Its prime factorisation is
- 3
The divisors add up to 21712; without 18991 itself that is 2721, so 18991 is deficient (2721 is less than 18991). Euler's totient φ(18991) = 16272 numbers up to 18991 share no factor with it.
- 4
The digits of 18991 add up to 28, and repeating that until one digit is left gives the digital root 1. The Collatz sequence (halve if even, else 3n+1) takes 92 steps to reach 1.
LaTeX source of the formulas on this page
\[ 18991 = 7 \times 2713 \]
18991 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 21712 |
| Euler totient φ(n) | 16272 |
| Square | 360658081 |
| Cube | 6849257616271 |
| Square root | 137.807837 |
| Digit sum / digital root | 28 / 1 |
| Collatz steps to 1 | 92 |
| Scientific notation | 1.8991 × |
18991 in other forms
| Form | Value |
|---|---|
| In words | eighteen thousand nine hundred and ninety-one |
| Binary (base 2) | 100101000101111 |
| Octal (base 8) | 45057 |
| Hexadecimal (base 16) | 4A2F |
| Base 3 | 222001101 |
| As time | 18991 seconds = 5 h 16 min 31 s |
Is 18991 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 | Yes (18991 ÷ 7 = 2713) | 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 1) | 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 5) | 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 18991 a prime number?
No. 18991 = 7 × 2713, so it is divisible by 7.
What are the factors of 18991?
1, 7, 2713, 18991.
What is 18991 in binary?
100101000101111 (hexadecimal 4A2F, octal 45057).
What is the square root of 18991?
About 137.8078.
Is 18991 divisible by 3?
No. Its digits add up to 28, which is not a multiple of 3.
What is special about the number 18991?
18991 is a semiprime: 7 × 2713, the product of two primes.