In logic, mathematics and computer science, especially metalogic and computability theory, an effective method or effective procedure is a procedure for solving a problem from a specific class. An effective method is sometimes also called a mechanical method or procedure.
The definition of an effective method involves more than the method itself. In order for a method to be called effective, it must be considered with respect to a class of problems. Because of this, one method may be effective with respect to one class of problems and not be effective with respect to a different class.
A method is formally called effective for a class of problems when it satisfies these criteria:
Optionally, it may also be required that the method never returns a result as if it were an answer when the method is applied to a problem from outside its class. Adding this requirement reduces the set of classes for which there is an effective method.
An effective method for calculating the values of a function is an algorithm. Functions for which an effective method exists are sometimes called effectively calculable.
Several independent efforts to give a formal characterization of effective calculability led to a variety of proposed definitions (general recursion, Turing machines, ?-calculus) that later were shown to be equivalent. The notion captured by these definitions is known as recursive or effective computability.
The Church-Turing thesis states that the two notions coincide: any number-theoretic function that is effectively calculable is recursively computable. As this is not a mathematical statement, it cannot be proven by a mathematical proof.