The number 861
861 is a triangular number: 1 + 2 + … + 41 = 861. Its prime factorisation is 3 × 7 × 41. Divisors (8): 1, 3, 7, 21, 41, 123, 287, 861.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 861: a triangular number: 1 + 2 + … + 41 = 861.
- 2
861 is composite. Its prime factorisation is
- 3
The divisors add up to 1344; without 861 itself that is 483, so 861 is deficient (483 is less than 861). Euler's totient φ(861) = 480 numbers up to 861 share no factor with it.
- 4
The digits of 861 add up to 15, and repeating that until one digit is left gives the digital root 6. The Collatz sequence (halve if even, else 3n+1) takes 103 steps to reach 1.
LaTeX source of the formulas on this page
\[ 861 = 3 \times 7 \times 41 \]
861 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 1344 |
| Euler totient φ(n) | 480 |
| Square | 741321 |
| Cube | 638277381 |
| Square root | 29.342802 |
| Digit sum / digital root | 15 / 6 |
| Collatz steps to 1 | 103 |
| Scientific notation | 8.61 × |
861 in other forms
| Form | Value |
|---|---|
| In words | eight hundred and sixty-one |
| Roman numerals | DCCCLXI |
| Binary (base 2) | 1101011101 |
| Octal (base 8) | 1535 |
| Hexadecimal (base 16) | 35D |
| Base 3 | 1011220 |
| As time | 861 seconds = 14 min 21 s |
Is 861 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 (861 ÷ 3 = 287) | 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 | Yes (861 ÷ 7 = 123) | 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 6) | 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 9) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 861 a prime number?
No. 861 = 3 × 7 × 41, so it is divisible by 3.
What are the factors of 861?
1, 3, 7, 21, 41, 123, 287, 861.
What is 861 in binary?
1101011101 (hexadecimal 35D, octal 1535).
What is the square root of 861?
About 29.3428.
What is 861 in Roman numerals?
DCCCLXI.
Is 861 divisible by 3?
Yes. Its digits add up to 15, a multiple of 3.
What is special about the number 861?
861 is a triangular number: 1 + 2 + … + 41 = 861.