The number 21985
21985 is a composite number with 4 divisors. Its prime factorisation is 5 × 4397. Divisors (4): 1, 5, 4397, 21985.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21985: a semiprime: 5 × 4397, the product of two primes.
- 2
21985 is composite. Its prime factorisation is
- 3
The divisors add up to 26388; without 21985 itself that is 4403, so 21985 is deficient (4403 is less than 21985). Euler's totient φ(21985) = 17584 numbers up to 21985 share no factor with it.
- 4
The digits of 21985 add up to 25, and repeating that until one digit is left gives the digital root 7. The Collatz sequence (halve if even, else 3n+1) takes 162 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21985 = 5 \times 4397 \]
21985 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 26388 |
| Euler totient φ(n) | 17584 |
| Square | 483340225 |
| Cube | 10626234846625 |
| Square root | 148.273396 |
| Digit sum / digital root | 25 / 7 |
| Collatz steps to 1 | 162 |
| Scientific notation | 2.1985 × |
21985 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand nine hundred and eighty-five |
| Binary (base 2) | 101010111100001 |
| Octal (base 8) | 52741 |
| Hexadecimal (base 16) | 55E1 |
| Base 3 | 1010011021 |
| As time | 21985 seconds = 6 h 6 min 25 s |
Is 21985 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 | Yes (21985 ÷ 5 = 4397) | 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 5) | 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 7) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 5) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 7) | 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 21985 a prime number?
No. 21985 = 5 × 4397, so it is divisible by 5.
What are the factors of 21985?
1, 5, 4397, 21985.
What is 21985 in binary?
101010111100001 (hexadecimal 55E1, octal 52741).
What is the square root of 21985?
About 148.2734.
Is 21985 divisible by 3?
No. Its digits add up to 25, which is not a multiple of 3.
What is special about the number 21985?
21985 is a semiprime: 5 × 4397, the product of two primes.