The number 81821
81821 is a composite number with 4 divisors. Its prime factorisation is 17 × 4813. Divisors (4): 1, 17, 4813, 81821.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 81821: a semiprime: 17 × 4813, the product of two primes.
- 2
81821 is composite. Its prime factorisation is
- 3
The divisors add up to 86652; without 81821 itself that is 4831, so 81821 is deficient (4831 is less than 81821). Euler's totient φ(81821) = 76992 numbers up to 81821 share no factor with it.
- 4
The digits of 81821 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 37 steps to reach 1.
LaTeX source of the formulas on this page
\[ 81821 = 17 \times 4813 \]
81821 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 86652 |
| Euler totient φ(n) | 76992 |
| Square | 6694676041 |
| Cube | 547765088350661 |
| Square root | 286.043703 |
| Digit sum / digital root | 20 / 2 |
| Collatz steps to 1 | 37 |
| Scientific notation | 8.1821 × |
81821 in other forms
| Form | Value |
|---|---|
| In words | eighty-one thousand eight hundred and twenty-one |
| Binary (base 2) | 10011111110011101 |
| Octal (base 8) | 237635 |
| Hexadecimal (base 16) | 13F9D |
| Base 3 | 11011020102 |
| As time | 81821 seconds = 22 h 43 min 41 s |
Is 81821 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 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 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 3) | 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 81821 a prime number?
No. 81821 = 17 × 4813, so it is divisible by 17.
What are the factors of 81821?
1, 17, 4813, 81821.
What is 81821 in binary?
10011111110011101 (hexadecimal 13F9D, octal 237635).
What is the square root of 81821?
About 286.0437.
Is 81821 divisible by 3?
No. Its digits add up to 20, which is not a multiple of 3.
What is special about the number 81821?
81821 is a semiprime: 17 × 4813, the product of two primes.