Effective method

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Effective method" – news · newspapers · books · scholar · JSTOR (May 2025) (Learn how and when to remove this message)

In metalogic, mathematical logic, and computability theory, an effective method^[1] or effective procedure is a finite-time, deterministic procedure for solving a problem from a specific class.^[2][3] An effective method is sometimes also called a mechanical method or procedure.^[4]

Formally, a method is called effective to a specific class of problems when it satisfies the following criteria:

• It consists of a finite number of exact, finite instructions.
• When it is applied to a problem from its class:
  • It always finishes (terminates) after a finite number of steps.
  • It always produces a correct answer.
• In principle, it can be done by a human without any aids except writing materials.
• Its instructions need only to be followed rigorously to succeed. In other words, it requires no ingenuity to succeed.^[5]

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 recursive functions, Turing machines, λ-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.^{[citation needed]}

• Decidability (logic)
• Decision problem
• Effective results in number theory
• Function problem
• Model of computation
• Recursive set
• Undecidable problem

• S. C. Kleene (1967), Mathematical logic. Reprinted, Dover, 2002, ISBN 0-486-42533-9, pp. 233 ff., esp. p. 231.