- 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
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 a German club requesting an end to this practice.
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