"One of the most powerful methods of proof — and one of the most difficult to wrap your head around — is called mathematical induction, or just “induction” for short. I like to call it “proof by recursion,” because this is exactly what it is. Remember that we discussed recursion in the context of rooted trees (see p.114). A tree can be thought of as a node with several children — each of which are, in turn, trees. Each of them is the root node of a tree comprised of yet smaller trees, and so on and so forth. If you flip back to the left-hand side of Figure 5.16 on p.111, you’ll see that A is the root of one tree, and its two children, F and B, are roots of their own smaller trees in turn. If we were to traverse this tree in (say) pre-order, we’d visit the root, then visit the left and right subtrees in turn, treating each of them as their own tree. In this way we’ve broken up a larger problem (traversing the big tree) into smaller problems (traversing the smaller trees F and B). The A node has very little to do: it just visits itself, then defers all the rest of the work onto its children. This idea of pawning off most of the work onto smaller subproblems that you trust will work is key to the idea of inductive proofs. Mathematical induction is hard to wrap your head around because it feels like cheating. It seems like you never actually prove anything: you defer all the work to someone else, and then declare victory. But the chain of reasoning, though delicate, is strong as iron."