The number 21729
21729 is a composite number with 4 divisors. Its prime factorisation is 3 × 7243. Divisors (4): 1, 3, 7243, 21729.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21729: a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 7243, the product of two primes.
- 2
21729 is composite. Its prime factorisation is
- 3
The divisors add up to 28976; without 21729 itself that is 7247, so 21729 is deficient (7247 is less than 21729). Euler's totient φ(21729) = 14484 numbers up to 21729 share no factor with it.
- 4
The digits of 21729 add up to 21, and repeating that until one digit is left gives the digital root 3. The Collatz sequence (halve if even, else 3n+1) takes 100 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21729 = 3 \times 7243 \]
21729 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 28976 |
| Euler totient φ(n) | 14484 |
| Square | 472149441 |
| Cube | 10259335203489 |
| Square root | 147.407598 |
| Digit sum / digital root | 21 / 3 |
| Collatz steps to 1 | 100 |
| Scientific notation | 2.1729 × |
21729 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand seven hundred and twenty-nine |
| Binary (base 2) | 101010011100001 |
| Octal (base 8) | 52341 |
| Hexadecimal (base 16) | 54E1 |
| Base 3 | 1002210210 |
| As time | 21729 seconds = 6 h 2 min 9 s |
Is 21729 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 | Yes (21729 ÷ 3 = 7243) | 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 4) | A number is divisible by 5 if the last digit is 0 or 5. |
| 6 | No (remainder 3) | 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 1) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 3) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 9) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 4) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 9) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21729 a prime number?
No. 21729 = 3 × 7243, so it is divisible by 3.
What are the factors of 21729?
1, 3, 7243, 21729.
What is 21729 in binary?
101010011100001 (hexadecimal 54E1, octal 52341).
What is the square root of 21729?
About 147.4076.
Is 21729 divisible by 3?
Yes. Its digits add up to 21, a multiple of 3.
What is special about the number 21729?
21729 is a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 7243, the product of two primes.