The number 1981
1981 is a composite number with 4 divisors. Its prime factorisation is 7 × 283. Divisors (4): 1, 7, 283, 1981.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 1981: a semiprime: 7 × 283, the product of two primes.
- 2
1981 is composite. Its prime factorisation is
- 3
The divisors add up to 2272; without 1981 itself that is 291, so 1981 is deficient (291 is less than 1981). Euler's totient φ(1981) = 1692 numbers up to 1981 share no factor with it.
- 4
The digits of 1981 add up to 19, and repeating that until one digit is left gives the digital root 1. The Collatz sequence (halve if even, else 3n+1) takes 99 steps to reach 1.
LaTeX source of the formulas on this page
\[ 1981 = 7 \times 283 \]
1981 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 2272 |
| Euler totient φ(n) | 1692 |
| Square | 3924361 |
| Cube | 7774159141 |
| Square root | 44.508426 |
| Digit sum / digital root | 19 / 1 |
| Collatz steps to 1 | 99 |
| Scientific notation | 1.981 × |
1981 in other forms
| Form | Value |
|---|---|
| In words | one thousand nine hundred and eighty-one |
| Roman numerals | MCMLXXXI |
| Binary (base 2) | 11110111101 |
| Octal (base 8) | 3675 |
| Hexadecimal (base 16) | 7BD |
| Base 3 | 2201101 |
| As time | 1981 seconds = 33 min 1 s |
Is 1981 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 (1981 ÷ 7 = 283) | 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 1) | 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 1) | 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 1981 a prime number?
No. 1981 = 7 × 283, so it is divisible by 7.
What are the factors of 1981?
1, 7, 283, 1981.
What is 1981 in binary?
11110111101 (hexadecimal 7BD, octal 3675).
What is the square root of 1981?
About 44.5084.
What is 1981 in Roman numerals?
MCMLXXXI.
Is 1981 divisible by 3?
No. Its digits add up to 19, which is not a multiple of 3.
What is special about the number 1981?
1981 is a semiprime: 7 × 283, the product of two primes.