The number 665
665 is a composite number with 8 divisors. Its prime factorisation is 5 × 7 × 19. Divisors (8): 1, 5, 7, 19, 35, 95, 133, 665.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 665: a happy number: repeatedly adding the squares of its digits reaches 1.
- 2
665 is composite. Its prime factorisation is
- 3
The divisors add up to 960; without 665 itself that is 295, so 665 is deficient (295 is less than 665). Euler's totient φ(665) = 432 numbers up to 665 share no factor with it.
- 4
The digits of 665 add up to 17, and repeating that until one digit is left gives the digital root 8. The Collatz sequence (halve if even, else 3n+1) takes 51 steps to reach 1.
LaTeX source of the formulas on this page
\[ 665 = 5 \times 7 \times 19 \]
665 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 960 |
| Euler totient φ(n) | 432 |
| Square | 442225 |
| Cube | 294079625 |
| Square root | 25.787594 |
| Digit sum / digital root | 17 / 8 |
| Collatz steps to 1 | 51 |
| Scientific notation | 6.65 × |
665 in other forms
| Form | Value |
|---|---|
| In words | six hundred and sixty-five |
| Roman numerals | DCLXV |
| Binary (base 2) | 1010011001 |
| Octal (base 8) | 1231 |
| Hexadecimal (base 16) | 299 |
| Base 3 | 220122 |
| As time | 665 seconds = 11 min 5 s |
Is 665 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 | Yes (665 ÷ 5 = 133) | 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 | Yes (665 ÷ 7 = 95) | 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 1) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 8) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 5) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 5) | 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 665 a prime number?
No. 665 = 5 × 7 × 19, so it is divisible by 5.
What are the factors of 665?
1, 5, 7, 19, 35, 95, 133, 665.
What is 665 in binary?
1010011001 (hexadecimal 299, octal 1231).
What is the square root of 665?
About 25.7876.
What is 665 in Roman numerals?
DCLXV.
Is 665 divisible by 3?
No. Its digits add up to 17, which is not a multiple of 3.
What is special about the number 665?
665 is a happy number: repeatedly adding the squares of its digits reaches 1.