21 (twenty-one) is a positive integer following 20 and preceding 22. It is an odd composite number, and the 6th triangular number

In googology Edit

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 21Edit


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 a German club requesting an end to this practice.