# Online Encyclopedia

# Automated theorem proving

**Automated theorem proving** (the currently most important subfield of *automated reasoning*) is the proving of mathematical theorems by a computer program. Depending on the underlying logic, the problem of deciding the validity of a theorem varies from trivial to impossible. For the frequent case of propositional logic, the problem is NP-complete, and hence only exponential algorithms are believed to exist. For first-order logic it is recursively enumerable, i.e., given unbounded resources, any true theorem can eventually be proven, but invalid theorems cannot always be recognized. Despite these theoretical limits, practical theorem provers can solve many hard problems.

A simpler, but related problem is proof verification , where an existing proof for a theorem is certified valid. For this, it is generally required that each individual proof step can be verified by a primitive recursive function or program, and hence the problem is always decidable.

*Interactive theorem provers* require a human user to give hints to the system. Depending on the degree of automation, the prover can essentially be reduced to a proof checker, with the user providing the proof in a formal way, or significant proof tasks can be performed automatically. Interactive provers are used for a variety of tasks, but even fully automatic systems have by now proven a number of interesting and hard theorems, including some that have eluded human mathematicians for a long time. However, these successes are sporadic, and work on hard problems usually requires a proficient user.

Another distinction is sometimes drawn between theorem proving and other techniques, where a process is considered to be theorem proving if it consists of a traditional proof, starting with axioms and producing new inference steps using rules of inference. Other techniques would include model checking, which is equivalent to brute-force enumeration of many possible states (although the actual implementation of model checkers requires much cleverness, and does not simply reduce to brute force). There are hybrid theorem proving systems which use model checking as an inference rule. There are also programs which were written to prove a particular theorem, with a (usually informal) proof that if the program finishes with a certain result, then the theorem is true. A good example of this was the machine-aided proof of the four color theorem, which was very controversial as the first claimed mathematical proof which was essentially impossible to check by hand. Another example would be the proof that the game Connect Four is a win for the first player.

Commercial use of automated theorem proving is mostly concentrated in integrated circuit design and verification. Since the Pentium FDIV bug, the complicated floating point units of modern microprocessors have been designed with extra scrutiny. In the latest processors from AMD, Intel, and others, automated theorem proving has been used to verify that the divide and other operations are correct.

## Popular techniques

- First-order resolution
- Term rewriting
- Model checking
- Mathematical induction
- Binary decision diagrams
- Unification
- Higher-order unification

## Popular implementations

Some modern theorem provers:

- Isabelle
- HOL
- Paradox
- ACL2
- PVS
- Simplify
- Otter
- Vampire
- Waldmeister
- E
- SPASS
- Gandalf
- NuPRL
- MetaPRL
- Coq
- Mizar
- CVC Lite

You can find information on some of these theorem provers and others at http://www.cs.miami.edu/~tptp/CASC/19/SystemDescriptions.html

## Important people

- Leo Bachmair
- Robert Stephen Boyer
- Bill McCune
- Martin Davis
- Harald Ganzinger
- Michael Genesereth
- Donald W. Loveland
- Sergei Maslov
- J Strother Moore
- Robert Nieuwenhuis
- Ross Overbeek
- David A. Plaisted
- John Rushby
- Alan Robinson
- Robert Veroff
- Andrei Voronkov
- Larry Wos