21

21 (twenty-one) is a positive integer following 20 and preceding 22. It is an , and the 6th . It can be also called long score.https://books.google.cz/books?id=cDkSAAAAYAAJ&pg=PA417&lpg=PA417&dq=%22long+score%22+21&source=bl&ots=uU-HfR9K0J&sig=YhXx-SlxYVF38x27a_X9Ia7ncR8&hl=en&ei=9vjSTbPvM8ezrAeys6jECQ&sa=X&oi=book_result&ct=result&redir_esc=y#v=onepage&q&f=false 
== Properties ==
21 is the smallest amounts of different size of squares to fill a larger square.MathWorld, Perfect Square DissectionDuijvestijn, A. J. W. A Simple Perfect Square of Lowest Order. J. Combin. Th. Ser. B 25, 240-243, 1978.
== In googology ==
*In Greek-based number-naming systems, 21 is associated with prefix "henicosa-", and with prefix "unviginti-" in Latin systems.
*It is equal to S(3) with the S function, a variant of the Busy beaver function. This means 21 is the maximum finite number of steps that can be performed using a 3-state, 2-color Turing machine.
===Googological functions returning 21===
* weak Goodstein function: \(g(4)=21\)
* Maximum shifts function: \(S(3)=21\)
== In other languages ==
In languages such as Dutch and German, the ones are put before the tens; the resulting names are eenentwintig and einundzwanzig, respectively. There is requesting an end to this practice.
== Source ==
Category:Numbers
Category:Class 1
Category:Fibonacci numbers
Category:Non-powers
Category:Numbers with 1 to 2 digits
Category:Odd numbers