The number 21721
21721 is a composite number with 8 divisors. Its prime factorisation is 7 × 29 × 107. Divisors (8): 1, 7, 29, 107, 203, 749, 3103, 21721.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
21721 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
21721 is composite. Its prime factorisation is
- 3
The divisors add up to 25920; without 21721 itself that is 4199, so 21721 is deficient (4199 is less than 21721). Euler's totient φ(21721) = 17808 numbers up to 21721 share no factor with it.
- 4
The digits of 21721 add up to 13, and repeating that until one digit is left gives the digital root 4. The Collatz sequence (halve if even, else 3n+1) takes 162 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21721 = 7 \times 29 \times 107 \]
21721 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 25920 |
| Euler totient φ(n) | 17808 |
| Square | 471801841 |
| Cube | 10248007788361 |
| Square root | 147.380460 |
| Digit sum / digital root | 13 / 4 |
| Collatz steps to 1 | 162 |
| Scientific notation | 2.1721 × |
21721 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand seven hundred and twenty-one |
| Binary (base 2) | 101010011011001 |
| Octal (base 8) | 52331 |
| Hexadecimal (base 16) | 54D9 |
| Base 3 | 1002210111 |
| As time | 21721 seconds = 6 h 2 min 1 s |
Is 21721 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 (21721 ÷ 7 = 3103) | 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 4) | 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 7) | 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 21721 a prime number?
No. 21721 = 7 × 29 × 107, so it is divisible by 7.
What are the factors of 21721?
1, 7, 29, 107, 203, 749, 3103, 21721.
What is 21721 in binary?
101010011011001 (hexadecimal 54D9, octal 52331).
What is the square root of 21721?
About 147.3805.
Is 21721 divisible by 3?
No. Its digits add up to 13, which is not a multiple of 3.
What is special about the number 21721?
It is a composite number with 8 divisors.