Busy beavers are special Turing machines that write as many ones as possible to the tape and which enter the halt state (i.e. stop) after a finite number of calculation steps.
The Radó function (also known as the busy beaver function) specifies the maximum number of ones that a busy beaver can write with a given number of states. Both were first considered in 1962 by the Hungarian mathematician Tibor Radó.
The following numbers are currently known:
| States | Steps |
| 1 | 1 |
| 2 | 6 |
| 3 | 21 |
| 4 | 107 |
| 5 | 47176870 |