The number 7821
7821 is a composite number with 12 divisors. Its prime factorisation is × 11 × 79. Divisors (12): 1, 3, 9, 11, 33, 79, 99, 237, 711, 869, 2607, 7821.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
7821 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
7821 is composite. Its prime factorisation is
- 3
The divisors add up to 12480; without 7821 itself that is 4659, so 7821 is deficient (4659 is less than 7821). Euler's totient φ(7821) = 4680 numbers up to 7821 share no factor with it.
- 4
The digits of 7821 add up to 18, and repeating that until one digit is left gives the digital root 9. The Collatz sequence (halve if even, else 3n+1) takes 101 steps to reach 1.
LaTeX source of the formulas on this page
\[ 7821 = 3^{2} \times 11 \times 79 \]7821 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 12 |
| Sum of divisors | 12480 |
| Euler totient φ(n) | 4680 |
| Square | 61168041 |
| Cube | 478395248661 |
| Square root | 88.436418 (= 3√869) |
| Digit sum / digital root | 18 / 9 |
| Collatz steps to 1 | 101 |
| Scientific notation | 7.821 × |
7821 in other forms
| Form | Value |
|---|---|
| In words | seven thousand eight hundred and twenty-one |
| Binary (base 2) | 1111010001101 |
| Octal (base 8) | 17215 |
| Hexadecimal (base 16) | 1E8D |
| Base 3 | 101201200 |
| As time | 7821 seconds = 2 h 10 min 21 s |
Is 7821 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 (7821 ÷ 3 = 2607) | 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 2) | 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 | Yes (7821 ÷ 9 = 869) | 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 | Yes (7821 ÷ 11 = 711) | 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 7821 a prime number?
No. 7821 = × 11 × 79, so it is divisible by 3.
What are the factors of 7821?
1, 3, 9, 11, 33, 79, 99, 237, 711, 869, 2607, 7821.
What is 7821 in binary?
1111010001101 (hexadecimal 1E8D, octal 17215).
What is the square root of 7821?
About 88.4364, exactly 3√869.
Is 7821 divisible by 3?
Yes. Its digits add up to 18, a multiple of 3.
What is special about the number 7821?
It is a composite number with 12 divisors.