**Computable Functions**

- The Busy Beaver functions
- Wikipedia’s page
- Mathword's Page
- Heiner Marxen’s page with the current status of many efforts to determine values for
*busy-beaverones*(Rado’s S function) - Michael Somos’s page with a bibliography
- The quadruple version of the problem (in which a Turing machine may either write o
- Historical survey of busy beaver results for several variants of the problem

**Recursive Function Theory**

- Ackermann’s function

**The Recursion Theorem and its Use**