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Today is Day 21 of 2018

21

star numberis the 3rd star number (3 x 3 + 4 x 3).
is a Fibonacci number, and the smallest number of differently sized squares that are needed to tile a square.
is the number of spots on a standard cubical die (1+2+3+4+5+6).
is the number of ways of choosing a set of 5 elements from a set of 7 elements.
is x for: y=2x-22 and 3x-4y+17=0

= 112 - 102 = 52 - 22
= 40 + 41 + 42
= 62 - 52 + 42 - 32 + 22 - 12

If a square number ends with xyxyxyxyxy, then xy = 21, 29, 61, 69 or 84. For instance: 508,853,9892 = 258932382121212121

221 - 21 is prime.

The smallest simple perfect squared square (discovered by A. J. W. Duijvestijn in 1976 - Journal of Combination Theory) is tiled using 21 different squares.
squared square

"When I Was One-and-Twenty" is a dramatical verse by Alfred Edward Housman about the futility of love, youth, experience, and the irony in living life.

'Twenty-one' is the name of a card game (also called 'blackjack').

the German 'Twenty-One Club' (Zwanzigeins e. V.) is dedicated to the promotion of a new way to spell numbers in order to make mental arithmetic easier. The supporters of the club want Germans to say zwanzigeins (twenty-one) instead of the current einundzwanzig (one-and-twenty).

Many thanks are extended to Archimedes' Lab for their information.




21

Today is day number   of 2018. Below are various representations of the number 21, some dating thousands of years, and some very modern.

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Chinese

Egyptian

Mayan

 

Babylonian

Greek

Abacus

Roman

 

Hex

 

Binary

 

Many thanks are extended to Jo Edkins for her wonderful work.

 



 

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