The number 2821
2821 is a composite number with 8 divisors. Its prime factorisation is 7 × 13 × 31. Divisors (8): 1, 7, 13, 31, 91, 217, 403, 2821.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 2821: a Harshad number: divisible by its digit sum 13.
- 2
2821 is composite. Its prime factorisation is
- 3
The divisors add up to 3584; without 2821 itself that is 763, so 2821 is deficient (763 is less than 2821). Euler's totient φ(2821) = 2160 numbers up to 2821 share no factor with it.
- 4
The digits of 2821 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 35 steps to reach 1.
LaTeX source of the formulas on this page
\[ 2821 = 7 \times 13 \times 31 \]
2821 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 3584 |
| Euler totient φ(n) | 2160 |
| Square | 7958041 |
| Cube | 22449633661 |
| Square root | 53.113087 |
| Digit sum / digital root | 13 / 4 |
| Collatz steps to 1 | 35 |
| Scientific notation | 2.821 × |
2821 in other forms
| Form | Value |
|---|---|
| In words | two thousand eight hundred and twenty-one |
| Roman numerals | MMDCCCXXI |
| Binary (base 2) | 101100000101 |
| Octal (base 8) | 5405 |
| Hexadecimal (base 16) | B05 |
| Base 3 | 10212111 |
| As time | 2821 seconds = 47 min 1 s |
Is 2821 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 (2821 ÷ 7 = 403) | 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 5) | 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 2821 a prime number?
No. 2821 = 7 × 13 × 31, so it is divisible by 7.
What are the factors of 2821?
1, 7, 13, 31, 91, 217, 403, 2821.
What is 2821 in binary?
101100000101 (hexadecimal B05, octal 5405).
What is the square root of 2821?
About 53.1131.
What is 2821 in Roman numerals?
MMDCCCXXI.
Is 2821 divisible by 3?
No. Its digits add up to 13, which is not a multiple of 3.
What is special about the number 2821?
2821 is a Harshad number: divisible by its digit sum 13.