The number 781
781 is a composite number with 4 divisors. Its prime factorisation is 11 × 71. Divisors (4): 1, 11, 71, 781.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 781: a semiprime: 11 × 71, the product of two primes.
- 2
781 is composite. Its prime factorisation is
- 3
The divisors add up to 864; without 781 itself that is 83, so 781 is deficient (83 is less than 781). Euler's totient φ(781) = 700 numbers up to 781 share no factor with it.
- 4
The digits of 781 add up to 16, and repeating that until one digit is left gives the digital root 7. The Collatz sequence (halve if even, else 3n+1) takes 121 steps to reach 1.
LaTeX source of the formulas on this page
\[ 781 = 11 \times 71 \]
781 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 864 |
| Euler totient φ(n) | 700 |
| Square | 609961 |
| Cube | 476379541 |
| Square root | 27.946377 |
| Digit sum / digital root | 16 / 7 |
| Collatz steps to 1 | 121 |
| Scientific notation | 7.81 × |
781 in other forms
| Form | Value |
|---|---|
| In words | seven hundred and eighty-one |
| Roman numerals | DCCLXXXI |
| Binary (base 2) | 1100001101 |
| Octal (base 8) | 1415 |
| Hexadecimal (base 16) | 30D |
| Base 3 | 1001221 |
| As time | 781 seconds = 13 min 1 s |
Is 781 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 4) | 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 7) | 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 | Yes (781 ÷ 11 = 71) | 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 781 a prime number?
No. 781 = 11 × 71, so it is divisible by 11.
What are the factors of 781?
1, 11, 71, 781.
What is 781 in binary?
1100001101 (hexadecimal 30D, octal 1415).
What is the square root of 781?
About 27.9464.
What is 781 in Roman numerals?
DCCLXXXI.
Is 781 divisible by 3?
No. Its digits add up to 16, which is not a multiple of 3.
What is special about the number 781?
781 is a semiprime: 11 × 71, the product of two primes.