The number 1821
1821 is a composite number with 4 divisors. Its prime factorisation is 3 × 607. Divisors (4): 1, 3, 607, 1821.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 1821: a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 607, the product of two primes.
- 2
1821 is composite. Its prime factorisation is
- 3
The divisors add up to 2432; without 1821 itself that is 611, so 1821 is deficient (611 is less than 1821). Euler's totient φ(1821) = 1212 numbers up to 1821 share no factor with it.
- 4
The digits of 1821 add up to 12, and repeating that until one digit is left gives the digital root 3. The Collatz sequence (halve if even, else 3n+1) takes 42 steps to reach 1.
LaTeX source of the formulas on this page
\[ 1821 = 3 \times 607 \]
1821 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 2432 |
| Euler totient φ(n) | 1212 |
| Square | 3316041 |
| Cube | 6038510661 |
| Square root | 42.673177 |
| Digit sum / digital root | 12 / 3 |
| Collatz steps to 1 | 42 |
| Scientific notation | 1.821 × |
1821 in other forms
| Form | Value |
|---|---|
| In words | one thousand eight hundred and twenty-one |
| Roman numerals | MDCCCXXI |
| Binary (base 2) | 11100011101 |
| Octal (base 8) | 3435 |
| Hexadecimal (base 16) | 71D |
| Base 3 | 2111110 |
| As time | 1821 seconds = 30 min 21 s |
Is 1821 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 (1821 ÷ 3 = 607) | 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 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 5) | 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 1) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 6) | 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 1821 a prime number?
No. 1821 = 3 × 607, so it is divisible by 3.
What are the factors of 1821?
1, 3, 607, 1821.
What is 1821 in binary?
11100011101 (hexadecimal 71D, octal 3435).
What is the square root of 1821?
About 42.6732.
What is 1821 in Roman numerals?
MDCCCXXI.
Is 1821 divisible by 3?
Yes. Its digits add up to 12, a multiple of 3.
What is special about the number 1821?
1821 is a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 607, the product of two primes.