The number 21521
21521 is a twin prime (21521 and 21523 are both prime) and the 2415th prime number. Its only divisors are 1 and 21521.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 21521: a twin prime (21521 and 21523 are both prime); the 2415th prime number.
- 2
21521 is prime, so its only factors are 1 and 21521. It is the 2415th prime; the next prime is 21523 and the one before is 21517.
- 3
The divisors add up to 21522; without 21521 itself that is 1, so 21521 is deficient (1 is less than 21521). Euler's totient φ(21521) = 21520 numbers up to 21521 share no factor with it.
- 4
The digits of 21521 add up to 11, and repeating that until one digit is left gives the digital root 2. The Collatz sequence (halve if even, else 3n+1) takes 74 steps to reach 1.
21521 at a glance
| Property | Value |
|---|---|
| Type | prime |
| Number of divisors | 2 |
| Sum of divisors | 21522 |
| Euler totient φ(n) | 21520 |
| Square | 463153441 |
| Cube | 9967525203761 |
| Square root | 146.700375 |
| Digit sum / digital root | 11 / 2 |
| Collatz steps to 1 | 74 |
| Scientific notation | 2.1521 × |
21521 in other forms
| Form | Value |
|---|---|
| In words | twenty-one thousand five hundred and twenty-one |
| Binary (base 2) | 101010000010001 |
| Octal (base 8) | 52021 |
| Hexadecimal (base 16) | 5411 |
| Base 3 | 1002112002 |
| As time | 21521 seconds = 5 h 58 min 41 s |
Is 21521 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 2) | 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 5) | 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 2) | 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 5) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 5) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 21521 a prime number?
Yes. 21521 is divisible only by 1 and itself.
What are the factors of 21521?
1, 21521.
What is 21521 in binary?
101010000010001 (hexadecimal 5411, octal 52021).
What is the square root of 21521?
About 146.7004.
Is 21521 divisible by 3?
No. Its digits add up to 11, which is not a multiple of 3.
What is special about the number 21521?
21521 is a twin prime (21521 and 21523 are both prime); the 2415th prime number.