The number 18721
18721 is a triangular number: 1 + 2 + … + 193 = 18721. Its prime factorisation is 97 × 193. Divisors (4): 1, 97, 193, 18721.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 18721: a triangular number: 1 + 2 + … + 193 = 18721; a semiprime: 97 × 193, the product of two primes.
- 2
18721 is composite. Its prime factorisation is
- 3
The divisors add up to 19012; without 18721 itself that is 291, so 18721 is deficient (291 is less than 18721). Euler's totient φ(18721) = 18432 numbers up to 18721 share no factor with it.
- 4
The digits of 18721 add up to 19, and repeating that until one digit is left gives the digital root 1. The Collatz sequence (halve if even, else 3n+1) takes 48 steps to reach 1.
LaTeX source of the formulas on this page
\[ 18721 = 97 \times 193 \]
18721 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 19012 |
| Euler totient φ(n) | 18432 |
| Square | 350475841 |
| Cube | 6561258219361 |
| Square root | 136.824705 |
| Digit sum / digital root | 19 / 1 |
| Collatz steps to 1 | 48 |
| Scientific notation | 1.8721 × |
18721 in other forms
| Form | Value |
|---|---|
| In words | eighteen thousand seven hundred and twenty-one |
| Binary (base 2) | 100100100100001 |
| Octal (base 8) | 44441 |
| Hexadecimal (base 16) | 4921 |
| Base 3 | 221200101 |
| As time | 18721 seconds = 5 h 12 min 1 s |
Is 18721 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 3) | 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 10) | 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 18721 a prime number?
No. 18721 = 97 × 193, so it is divisible by 97.
What are the factors of 18721?
1, 97, 193, 18721.
What is 18721 in binary?
100100100100001 (hexadecimal 4921, octal 44441).
What is the square root of 18721?
About 136.8247.
Is 18721 divisible by 3?
No. Its digits add up to 19, which is not a multiple of 3.
What is special about the number 18721?
18721 is a triangular number: 1 + 2 + … + 193 = 18721; a semiprime: 97 × 193, the product of two primes.