The number 261
261 is a composite number with 6 divisors. Its prime factorisation is × 29. Divisors (6): 1, 3, 9, 29, 87, 261.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 261: a Harshad number: divisible by its digit sum 9.
- 2
261 is composite. Its prime factorisation is
- 3
The divisors add up to 390; without 261 itself that is 129, so 261 is deficient (129 is less than 261). Euler's totient φ(261) = 168 numbers up to 261 share no factor with it.
- 4
The digits of 261 add up to 9, and repeating that until one digit is left gives the digital root 9. The Collatz sequence (halve if even, else 3n+1) takes 29 steps to reach 1.
LaTeX source of the formulas on this page
\[ 261 = 3^{2} \times 29 \]261 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 6 |
| Sum of divisors | 390 |
| Euler totient φ(n) | 168 |
| Square | 68121 |
| Cube | 17779581 |
| Square root | 16.155494 (= 3√29) |
| Digit sum / digital root | 9 / 9 |
| Collatz steps to 1 | 29 |
| Scientific notation | 2.61 × |
261 in other forms
| Form | Value |
|---|---|
| In words | two hundred and sixty-one |
| Roman numerals | CCLXI |
| Binary (base 2) | 100000101 |
| Octal (base 8) | 405 |
| Hexadecimal (base 16) | 105 |
| Base 3 | 100200 |
| As time | 261 seconds = 4 min 21 s |
Is 261 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 (261 ÷ 3 = 87) | 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 (261 ÷ 9 = 29) | 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 9) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 261 a prime number?
No. 261 = × 29, so it is divisible by 3.
What are the factors of 261?
1, 3, 9, 29, 87, 261.
What is 261 in binary?
100000101 (hexadecimal 105, octal 405).
What is the square root of 261?
About 16.1555, exactly 3√29.
What is 261 in Roman numerals?
CCLXI.
Is 261 divisible by 3?
Yes. Its digits add up to 9, a multiple of 3.
What is special about the number 261?
261 is a Harshad number: divisible by its digit sum 9.