Structural Induction

Structural induction is a proof method that is used in mathematical logic (e.g., in the proof of Łoś' theorem), computer science, graph theory, and some other mathematical fields. It is a generalization of mathematical induction. Structural recursion is a recursion method bearing the same relationship to structural induction as ordinary recursion bears to ordinary mathematical induction.

In general, the idea is that one wishes to prove some proposition P(x), where x is any instance of some sort of recursively defined structure such as lists or trees. A well-founded partial order is defined on the structures ("sublist" for lists and "subtree" for trees). The structural induction proof is a proof that the proposition holds for all the minimal structures, and that if it holds for the immediate substructures of a certain structure S, then it must hold for S also. (Formally speaking, this then satisfies the premises of an axiom of well-founded induction, which asserts that these two conditions are sufficient for the proposition to hold for all x.)

A structurally recursive function uses the same idea to define a recursive function: "base cases" handle each minimal structure and a rule for recursion. Structural recursion is usually proved correct by structural induction; in particularly easy cases, the inductive step is often left out. The length and ++ functions in the example below are structurally recursive.

For example, if the structures are lists, one usually introduces the partial order '<' in which L < M whenever list L is the tail of list M. Under this ordering, the empty list is the unique minimal element. A structural induction proof of some proposition P(l) then consists of two parts: A proof that P is true, and a proof that if P(L) is true for some list L, and if L is the tail of list M, then P(M) must also be true.

Eventually, there may exist more than one base case, and/or more than one inductive case, depending on how the function or structure was constructed. In those cases, a structural induction proof of some proposition P(l) then consists of: A) a proof that P(BC) is true for each base case BC, and B): a proof that if P(I) is true for some instance I, and M can be obtained from I by applying any one recursive rule once, then P(M) must also be true.