The number 21115
21115 is a triangular number: 1 + 2 + … + 205 = 21115. Its prime factorisation is 5 × 41 × 103. Divisors (8): 1, 5, 41, 103, 205, 515, 4223, 21115.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21115: a triangular number: 1 + 2 + … + 205 = 21115; a happy number: repeatedly adding the squares of its digits reaches 1.
- 2
21115 is composite. Its prime factorisation is
- 3
The divisors add up to 26208; without 21115 itself that is 5093, so 21115 is deficient (5093 is less than 21115). Euler's totient φ(21115) = 16320 numbers up to 21115 share no factor with it.
- 4
The digits of 21115 add up to 10, and repeating that until one digit is left gives the digital root 1. The Collatz sequence (halve if even, else 3n+1) takes 105 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21115 = 5 \times 41 \times 103 \]
21115 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 26208 |
| Euler totient φ(n) | 16320 |
| Square | 445843225 |
| Cube | 9413979695875 |
| Square root | 145.310013 |
| Digit sum / digital root | 10 / 1 |
| Collatz steps to 1 | 105 |
| Scientific notation | 2.1115 × |
21115 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand one hundred and fifteen |
| Binary (base 2) | 101001001111011 |
| Octal (base 8) | 51173 |
| Hexadecimal (base 16) | 527B |
| Base 3 | 1001222001 |
| As time | 21115 seconds = 5 h 51 min 55 s |
Is 21115 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 3) | A number is divisible by 4 if the last two digits make a multiple of 4. |
| 5 | Yes (21115 ÷ 5 = 4223) | 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 3) | 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 3) | 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 5) | 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 7) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21115 a prime number?
No. 21115 = 5 × 41 × 103, so it is divisible by 5.
What are the factors of 21115?
1, 5, 41, 103, 205, 515, 4223, 21115.
What is 21115 in binary?
101001001111011 (hexadecimal 527B, octal 51173).
What is the square root of 21115?
About 145.3100.
Is 21115 divisible by 3?
No. Its digits add up to 10, which is not a multiple of 3.
What is special about the number 21115?
21115 is a triangular number: 1 + 2 + … + 205 = 21115; a happy number: repeatedly adding the squares of its digits reaches 1.