The number 21

Representation numerals

Greek numeral

καʹ

Arabic numeral

٢١

Roman numeral

XXI

Maya numeral

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Chinese numeral

二十一

Hebrew numeral

כא
Numeral systems
Binary: 10101
Septimal (base 7): 30
Octal: 25
Hexadecimal: 15
Base 24: L
Factorization
21 = 3 × 7
21 has 4 divisors: 1, 3, 7, 21
Sum of all divisors other than 21: 11
Sums & differences
Difference of 2 squares: 21 = 52 - 22 = 112 - 102
Characteristics
21 is a Fibonacci number
21 is a triangular number
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The next triangular is: 28
21 is a octogonal number
21 is a palindromic number in binary system (10101)
Related primes
The 21th prime number is: 73
Previous prime number: 19 — Next prime number: 23

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