The number 661
661 is a twin prime (659 and 661 are both prime) and the 121st prime number. Its only divisors are 1 and 661.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 661: a twin prime (659 and 661 are both prime); the 121st prime number.
- 2
661 is prime, so its only factors are 1 and 661. It is the 121st prime; the next prime is 673 and the one before is 659.
- 3
The divisors add up to 662; without 661 itself that is 1, so 661 is deficient (1 is less than 661). Euler's totient φ(661) = 660 numbers up to 661 share no factor with it.
- 4
The digits of 661 add up to 13, and repeating that until one digit is left gives the digital root 4. The Collatz sequence (halve if even, else 3n+1) takes 113 steps to reach 1.
661 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 662 |
| Euler totient φ(n) | 660 |
| Square | 436921 |
| Cube | 288804781 |
| Square root | 25.709920 |
| Digit sum / digital root | 13 / 4 |
| Collatz steps to 1 | 113 |
| Scientific notation | 6.61 × |
661 in other forms
| Form | Value |
|---|---|
| In words | six hundred and sixty-one |
| Roman numerals | DCLXI |
| Binary (base 2) | 1010010101 |
| Octal (base 8) | 1225 |
| Hexadecimal (base 16) | 295 |
| Base 3 | 220111 |
| As time | 661 seconds = 11 min 1 s |
Is 661 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 1) | 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 1) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 3) | 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 4) | 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 1) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 1) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 661 a prime number?
Yes. 661 is divisible only by 1 and itself.
What are the factors of 661?
1, 661.
What is 661 in binary?
1010010101 (hexadecimal 295, octal 1225).
What is the square root of 661?
About 25.7099.
What is 661 in Roman numerals?
DCLXI.
Is 661 divisible by 3?
No. Its digits add up to 13, which is not a multiple of 3.
What is special about the number 661?
661 is a twin prime (659 and 661 are both prime); the 121st prime number.