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Twenty-one (21) is a positive integer one more than 20 and one less than 22.

PropertiesEdit

In googology Edit

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.

Sturen Zeran calls the number addiston, as expressed to 10+11 using the addition.

Googological functions returning 21Edit

File:21.gif

In other languages Edit

In languages such as Dutch and German, the ones are put before the tens; the resulting names are eenentwintig and einundzwanzig, respectively. There is Template:W requesting an end to this practice.