The number 21921
21921 is a composite number with 4 divisors. Its prime factorisation is 3 × 7307. Divisors (4): 1, 3, 7307, 21921.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21921: a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 7307, the product of two primes.
- 2
21921 is composite. Its prime factorisation is
- 3
The divisors add up to 29232; without 21921 itself that is 7311, so 21921 is deficient (7311 is less than 21921). Euler's totient φ(21921) = 14612 numbers up to 21921 share no factor with it.
- 4
The digits of 21921 add up to 15, and repeating that until one digit is left gives the digital root 6. The Collatz sequence (halve if even, else 3n+1) takes 193 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21921 = 3 \times 7307 \]
21921 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 29232 |
| Euler totient φ(n) | 14612 |
| Square | 480530241 |
| Cube | 10533703412961 |
| Square root | 148.057421 |
| Digit sum / digital root | 15 / 6 |
| Collatz steps to 1 | 193 |
| Scientific notation | 2.1921 × |
21921 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand nine hundred and twenty-one |
| Binary (base 2) | 101010110100001 |
| Octal (base 8) | 52641 |
| Hexadecimal (base 16) | 55A1 |
| Base 3 | 1010001220 |
| As time | 21921 seconds = 6 h 5 min 21 s |
Is 21921 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 (21921 ÷ 3 = 7307) | 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 3) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 4) | 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 6) | 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 9) | 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 21921 a prime number?
No. 21921 = 3 × 7307, so it is divisible by 3.
What are the factors of 21921?
1, 3, 7307, 21921.
What is 21921 in binary?
101010110100001 (hexadecimal 55A1, octal 52641).
What is the square root of 21921?
About 148.0574.
Is 21921 divisible by 3?
Yes. Its digits add up to 15, a multiple of 3.
What is special about the number 21921?
21921 is a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 7307, the product of two primes.