The number 721
721 is a composite number with 4 divisors. Its prime factorisation is 7 × 103. Divisors (4): 1, 7, 103, 721.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 721: a semiprime: 7 × 103, the product of two primes.
- 2
721 is composite. Its prime factorisation is
- 3
The divisors add up to 832; without 721 itself that is 111, so 721 is deficient (111 is less than 721). Euler's totient φ(721) = 612 numbers up to 721 share no factor with it.
- 4
The digits of 721 add up to 10, and repeating that until one digit is left gives the digital root 1. The Collatz sequence (halve if even, else 3n+1) takes 46 steps to reach 1.
LaTeX source of the formulas on this page
\[ 721 = 7 \times 103 \]
721 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 832 |
| Euler totient φ(n) | 612 |
| Square | 519841 |
| Cube | 374805361 |
| Square root | 26.851443 |
| Digit sum / digital root | 10 / 1 |
| Collatz steps to 1 | 46 |
| Scientific notation | 7.21 × |
721 in other forms
| Form | Value |
|---|---|
| In words | seven hundred and twenty-one |
| Roman numerals | DCCXXI |
| Binary (base 2) | 1011010001 |
| Octal (base 8) | 1321 |
| Hexadecimal (base 16) | 2D1 |
| Base 3 | 222201 |
| As time | 721 seconds = 12 min 1 s |
Is 721 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 | Yes (721 ÷ 7 = 103) | 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 1) | 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 1) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 721 a prime number?
No. 721 = 7 × 103, so it is divisible by 7.
What are the factors of 721?
1, 7, 103, 721.
What is 721 in binary?
1011010001 (hexadecimal 2D1, octal 1321).
What is the square root of 721?
About 26.8514.
What is 721 in Roman numerals?
DCCXXI.
Is 721 divisible by 3?
No. Its digits add up to 10, which is not a multiple of 3.
What is special about the number 721?
721 is a semiprime: 7 × 103, the product of two primes.