2981	 Subgoal Induction	 A proof method subgoal induction is presented as an alternative or supplement to the commonly used inductive assertion method. Its major virtue is that it can often be used to prove a loop s correctness directly from its input-output specification without the use of an invariant. The relation between subgoal induction and other commonly used induction rules is explored and in particular it is shown that subgoal induction can be viewed as a specialized form of computation induction. A set of sufficient conditions are presented which guarantee that an input-output specification is strong enough for the induction steps of a proof by subgoal induction to be valid. program verification proving programs correct induction rule computation induction inductive assertions structural induction proof rule recursive programs iterative programs
