The number 81991
81991 is a composite number with 16 divisors. Its prime factorisation is 7 × 13 × 17 × 53. Divisors (16): 1, 7, 13, 17, 53, 91, 119, 221, 371, 689, 901, 1547, 4823, 6307, 11713, 81991.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
81991 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
81991 is composite. Its prime factorisation is
- 3
The divisors add up to 108864; without 81991 itself that is 26873, so 81991 is deficient (26873 is less than 81991). Euler's totient φ(81991) = 59904 numbers up to 81991 share no factor with it.
- 4
The digits of 81991 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 138 steps to reach 1.
LaTeX source of the formulas on this page
\[ 81991 = 7 \times 13 \times 17 \times 53 \]
81991 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 16 |
| Sum of divisors | 108864 |
| Euler totient φ(n) | 59904 |
| Square | 6722524081 |
| Cube | 551186471925271 |
| Square root | 286.340706 |
| Digit sum / digital root | 28 / 1 |
| Collatz steps to 1 | 138 |
| Scientific notation | 8.1991 × |
81991 in other forms
| Form | Value |
|---|---|
| In words | eighty-one thousand nine hundred and ninety-one |
| Binary (base 2) | 10100000001000111 |
| Octal (base 8) | 240107 |
| Hexadecimal (base 16) | 14047 |
| Base 3 | 11011110201 |
| As time | 81991 seconds = 22 h 46 min 31 s |
Is 81991 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 (81991 ÷ 7 = 11713) | 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 8) | 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 81991 a prime number?
No. 81991 = 7 × 13 × 17 × 53, so it is divisible by 7.
What are the factors of 81991?
1, 7, 13, 17, 53, 91, 119, 221, 371, 689, 901, 1547, 4823, 6307, 11713, 81991.
What is 81991 in binary?
10100000001000111 (hexadecimal 14047, octal 240107).
What is the square root of 81991?
About 286.3407.
Is 81991 divisible by 3?
No. Its digits add up to 28, which is not a multiple of 3.
What is special about the number 81991?
It is a composite number with 16 divisors.