Structural induction

In mathematical logic, structural induction is a proof method that is used in computer science, graph theory, and some other mathematical fields.

Description

It is a generalization of mathematical induction over natural numbers, and can be further generalized to arbitrary Noetherian induction.

Structural recursion is a recursion method bearing the same relationship to structural induction as ordinary recursion bears to ordinary mathematical induction.

Structural induction is used to prove that some proposition P(x) holds for all x 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.

See also

• Coinduction
• Initial algebra
• Loop invariant, analog for loops
• Mathematical logic
• Recursion (computer science)

External links

• Structural induction @ Wikipedia.org