The number 20821
20821 is a composite number with 4 divisors. Its prime factorisation is 47 × 443. Divisors (4): 1, 47, 443, 20821.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 20821: a semiprime: 47 × 443, the product of two primes.
- 2
20821 is composite. Its prime factorisation is
- 3
The divisors add up to 21312; without 20821 itself that is 491, so 20821 is deficient (491 is less than 20821). Euler's totient φ(20821) = 20332 numbers up to 20821 share no factor with it.
- 4
The digits of 20821 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 30 steps to reach 1.
LaTeX source of the formulas on this page
\[ 20821 = 47 \times 443 \]
20821 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 21312 |
| Euler totient φ(n) | 20332 |
| Square | 433514041 |
| Cube | 9026195847661 |
| Square root | 144.294837 |
| Digit sum / digital root | 13 / 4 |
| Collatz steps to 1 | 30 |
| Scientific notation | 2.0821 × |
20821 in other forms
| Form | Value |
|---|---|
| In words | twenty thousand eight hundred and twenty-one |
| Binary (base 2) | 101000101010101 |
| Octal (base 8) | 50525 |
| Hexadecimal (base 16) | 5155 |
| Base 3 | 1001120011 |
| As time | 20821 seconds = 5 h 47 min 1 s |
Is 20821 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 5) | 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 9) | 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 20821 a prime number?
No. 20821 = 47 × 443, so it is divisible by 47.
What are the factors of 20821?
1, 47, 443, 20821.
What is 20821 in binary?
101000101010101 (hexadecimal 5155, octal 50525).
What is the square root of 20821?
About 144.2948.
Is 20821 divisible by 3?
No. Its digits add up to 13, which is not a multiple of 3.
What is special about the number 20821?
20821 is a semiprime: 47 × 443, the product of two primes.