The number 15821
15821 is a composite number with 4 divisors. Its prime factorisation is 13 × 1217. Divisors (4): 1, 13, 1217, 15821.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 15821: a semiprime: 13 × 1217, the product of two primes.
- 2
15821 is composite. Its prime factorisation is
- 3
The divisors add up to 17052; without 15821 itself that is 1231, so 15821 is deficient (1231 is less than 15821). Euler's totient φ(15821) = 14592 numbers up to 15821 share no factor with it.
- 4
The digits of 15821 add up to 17, and repeating that until one digit is left gives the digital root 8. The Collatz sequence (halve if even, else 3n+1) takes 146 steps to reach 1.
LaTeX source of the formulas on this page
\[ 15821 = 13 \times 1217 \]
15821 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 17052 |
| Euler totient φ(n) | 14592 |
| Square | 250304041 |
| Cube | 3960060232661 |
| Square root | 125.781557 |
| Digit sum / digital root | 17 / 8 |
| Collatz steps to 1 | 146 |
| Scientific notation | 1.5821 × |
15821 in other forms
| Form | Value |
|---|---|
| In words | fifteen thousand eight hundred and twenty-one |
| Binary (base 2) | 11110111001101 |
| Octal (base 8) | 36715 |
| Hexadecimal (base 16) | 3DCD |
| Base 3 | 210200222 |
| As time | 15821 seconds = 4 h 23 min 41 s |
Is 15821 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 2) | 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 5) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 1) | 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 8) | 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 3) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 5) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 15821 a prime number?
No. 15821 = 13 × 1217, so it is divisible by 13.
What are the factors of 15821?
1, 13, 1217, 15821.
What is 15821 in binary?
11110111001101 (hexadecimal 3DCD, octal 36715).
What is the square root of 15821?
About 125.7816.
Is 15821 divisible by 3?
No. Its digits add up to 17, which is not a multiple of 3.
What is special about the number 15821?
15821 is a semiprime: 13 × 1217, the product of two primes.