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

## PropertiesEdit

- 21 is a composite number, factoring into 3*7.
- 21 is the 8th Fibonacci number (between 13 and 34), and the 7th lucky number, between 15 and 25.
- 21 is the 6th triangular number (between 15 and 28).
- 21 is equal to 7!!!!, the quadruple factorial of 7.

## 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

- weak Goodstein function: \(g(4)=21\).

## 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.