Representation numerals

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XXI

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二十一

כא

Numeral systems

Binary: 10101

Septimal (base 7): 30

Octal: 25

Hexadecimal: 15

Base 24: L

Sums & differences

Difference of 2 squares: 21 = 5^{2} - 2^{2} = 11^{2} - 10^{2}

Characteristics

21 is a Fibonacci number

21 is a octogonal number

21 is a palindromic number in binary system (10101)