The number 15721
15721 is a composite number with 4 divisors. Its prime factorisation is 79 × 199. Divisors (4): 1, 79, 199, 15721.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 15721: a semiprime: 79 × 199, the product of two primes.
- 2
15721 is composite. Its prime factorisation is
- 3
The divisors add up to 16000; without 15721 itself that is 279, so 15721 is deficient (279 is less than 15721). Euler's totient φ(15721) = 15444 numbers up to 15721 share no factor with it.
- 4
The digits of 15721 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 84 steps to reach 1.
LaTeX source of the formulas on this page
\[ 15721 = 79 \times 199 \]
15721 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 16000 |
| Euler totient φ(n) | 15444 |
| Square | 247149841 |
| Cube | 3885442650361 |
| Square root | 125.383412 |
| Digit sum / digital root | 16 / 7 |
| Collatz steps to 1 | 84 |
| Scientific notation | 1.5721 × |
15721 in other forms
| Form | Value |
|---|---|
| In words | fifteen thousand seven hundred and twenty-one |
| Binary (base 2) | 11110101101001 |
| Octal (base 8) | 36551 |
| Hexadecimal (base 16) | 3D69 |
| Base 3 | 210120021 |
| As time | 15721 seconds = 4 h 22 min 1 s |
Is 15721 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 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 1) | 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 | No (remainder 2) | 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 15721 a prime number?
No. 15721 = 79 × 199, so it is divisible by 79.
What are the factors of 15721?
1, 79, 199, 15721.
What is 15721 in binary?
11110101101001 (hexadecimal 3D69, octal 36551).
What is the square root of 15721?
About 125.3834.
Is 15721 divisible by 3?
No. Its digits add up to 16, which is not a multiple of 3.
What is special about the number 15721?
15721 is a semiprime: 79 × 199, the product of two primes.