21
Everything about the number 21 — its names, how to write it in other bases and numeral systems, and what makes it interesting mathematically.
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Names of 21
| Nominal | 21 |
| Cardinal | twenty-one |
| Ordinal | 21st |
| This number as US currency | twenty-one dollars |
Properties (and some facts) of 21
| Is 21 an odd number? | 21 is an odd number |
| Is 21 an even number? | 21 is not an even number |
| Is 21 a palindrome? | 21 is not a palindrome |
| Is 21 a triangle number? | 21 is a triangle number |
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21 in other bases
| Base 2 (Binary) | 10101 |
| Base 3 (Ternary) | 210 |
| Base 4 (Quaternary) | 111 |
| Base 5 (Quinary) | 41 |
| Base 6 (Senary) | 33 |
| Base 8 (Octal) | 25 |
| Base 12 (Duodecimal) | 19 |
| Base 16 (Hexadecimal) | 15 |
| Base 20 (Vigesimal) | 11 |
Primes and factors tests for 21
| Is 21 a prime number? | 21 is not a prime number |
| Is 21 a perfect number? | 21 is not a perfect number |
| Is 21 a Mersenne prime? | 21 is not a Mersenne prime |
| Is 21 a Fermat prime? | 21 is not a Fermat prime |
| Is 21 a Fibonacci prime? | 21 is not a Fibonacci prime |
| Is 21 a partition prime? | 21 is not a partition prime |
| Is 21 a Pell prime? | 21 is not a Pell prime |
| Prime Factors | Prime factors of 21 |
21 in other numeral systems
| 21 in Roman numerals | XXI |
| 21 in Chinese numerals | 貳拾壹 |
| 21 in Babylonian numerals |  |
| 21 in Egyptian numerals | 𓎇𓏺 |
Roots and squares for 21
| Is 21 a square number? | 21 is not a square number |
| Is 21 a cube number? | 21 is not a cube number |
| Square root of 21 | 4.5825756949558 |
| Cube root of 21 | 2.7589241763811 |
Recreational math for 21
| Number 21 reversed | 12 |
| Sum of the digits | 3 |
| Number of digits | 2 |
Arithmetic tables for 21
Other pages about 21
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