The number 391
391 is a composite number with 4 divisors. Its prime factorisation is 17 × 23. Divisors (4): 1, 17, 23, 391.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 391: a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 17 × 23, the product of two primes.
- 2
391 is composite. Its prime factorisation is
- 3
The divisors add up to 432; without 391 itself that is 41, so 391 is deficient (41 is less than 391). Euler's totient φ(391) = 352 numbers up to 391 share no factor with it.
- 4
The digits of 391 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 120 steps to reach 1.
LaTeX source of the formulas on this page
\[ 391 = 17 \times 23 \]
391 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 432 |
| Euler totient φ(n) | 352 |
| Square | 152881 |
| Cube | 59776471 |
| Square root | 19.773720 |
| Digit sum / digital root | 13 / 4 |
| Collatz steps to 1 | 120 |
| Scientific notation | 3.91 × |
391 in other forms
| Form | Value |
|---|---|
| In words | three hundred and ninety-one |
| Roman numerals | CCCXCI |
| Binary (base 2) | 110000111 |
| Octal (base 8) | 607 |
| Hexadecimal (base 16) | 187 |
| Base 3 | 112111 |
| As time | 391 seconds = 6 min 31 s |
Is 391 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 3) | 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 6) | 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 7) | 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 6) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 7) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 391 a prime number?
No. 391 = 17 × 23, so it is divisible by 17.
What are the factors of 391?
1, 17, 23, 391.
What is 391 in binary?
110000111 (hexadecimal 187, octal 607).
What is the square root of 391?
About 19.7737.
What is 391 in Roman numerals?
CCCXCI.
Is 391 divisible by 3?
No. Its digits add up to 13, which is not a multiple of 3.
What is special about the number 391?
391 is a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 17 × 23, the product of two primes.