- 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.
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 a German club requesting an end to this practice.