The number 21129
21129 is a composite number with 4 divisors. Its prime factorisation is 3 × 7043. Divisors (4): 1, 3, 7043, 21129.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21129: a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 7043, the product of two primes.
- 2
21129 is composite. Its prime factorisation is
- 3
The divisors add up to 28176; without 21129 itself that is 7047, so 21129 is deficient (7047 is less than 21129). Euler's totient φ(21129) = 14084 numbers up to 21129 share no factor with it.
- 4
The digits of 21129 add up to 15, and repeating that until one digit is left gives the digital root 6. The Collatz sequence (halve if even, else 3n+1) takes 61 steps to reach 1.
LaTeX source of the formulas on this page
\[ 21129 = 3 \times 7043 \]
21129 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 28176 |
| Euler totient φ(n) | 14084 |
| Square | 446434641 |
| Cube | 9432717529689 |
| Square root | 145.358178 |
| Digit sum / digital root | 15 / 6 |
| Collatz steps to 1 | 61 |
| Scientific notation | 2.1129 × |
21129 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand one hundred and twenty-nine |
| Binary (base 2) | 101001010001001 |
| Octal (base 8) | 51211 |
| Hexadecimal (base 16) | 5289 |
| Base 3 | 1001222120 |
| As time | 21129 seconds = 5 h 52 min 9 s |
Is 21129 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 (21129 ÷ 3 = 7043) | 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 4) | 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 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 1) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 6) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 9) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 9) | 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 21129 a prime number?
No. 21129 = 3 × 7043, so it is divisible by 3.
What are the factors of 21129?
1, 3, 7043, 21129.
What is 21129 in binary?
101001010001001 (hexadecimal 5289, octal 51211).
What is the square root of 21129?
About 145.3582.
Is 21129 divisible by 3?
Yes. Its digits add up to 15, a multiple of 3.
What is special about the number 21129?
21129 is a happy number: repeatedly adding the squares of its digits reaches 1; a semiprime: 3 × 7043, the product of two primes.