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When is a function computable? The idea is that to say that a function $$f$$ is computable on input $$k$$ means that there is a sequence of easy steps that leads to the correct output $$f \left( k \right)$$. We will make this precise in Chapter 7, but roughly it means that one can start with some easy functions and build up the function $$f$$ by some relatively simple operations on previously defined functions.