The number 21505
21505 is a composite number with 16 divisors. Its prime factorisation is 5 × 11 × 17 × 23. Divisors (16): 1, 5, 11, 17, 23, 55, 85, 115, 187, 253, 391, 935, 1265, 1955, 4301, 21505.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
21505 has no famous special property in the usual lists (prime, square, cube, triangular, Fibonacci, palindrome, perfect), but it is still a composite number.
- 2
21505 is composite. Its prime factorisation is
- 3
The divisors add up to 31104; without 21505 itself that is 9599, so 21505 is deficient (9599 is less than 21505). Euler's totient φ(21505) = 14080 numbers up to 21505 share no factor with it.
- 4
The digits of 21505 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 74 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21505 = 5 \times 11 \times 17 \times 23 \]
21505 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 16 |
| Sum of divisors | 31104 |
| Euler totient φ(n) | 14080 |
| Square | 462465025 |
| Cube | 9945310362625 |
| Square root | 146.645832 |
| Digit sum / digital root | 13 / 4 |
| Collatz steps to 1 | 74 |
| Scientific notation | 2.1505 × |
21505 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand five hundred and five |
| Binary (base 2) | 101010000000001 |
| Octal (base 8) | 52001 |
| Hexadecimal (base 16) | 5401 |
| Base 3 | 1002111111 |
| As time | 21505 seconds = 5 h 58 min 25 s |
Is 21505 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 (21505 ÷ 5 = 4301) | 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 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 1) | 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 5) | A number is divisible by 10 if the last digit is 0. |
| 11 | Yes (21505 ÷ 11 = 1955) | 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 21505 a prime number?
No. 21505 = 5 × 11 × 17 × 23, so it is divisible by 5.
What are the factors of 21505?
1, 5, 11, 17, 23, 55, 85, 115, 187, 253, 391, 935, 1265, 1955, 4301, 21505.
What is 21505 in binary?
101010000000001 (hexadecimal 5401, octal 52001).
What is the square root of 21505?
About 146.6458.
Is 21505 divisible by 3?
No. Its digits add up to 13, which is not a multiple of 3.
What is special about the number 21505?
It is a composite number with 16 divisors.