Manyvalued logic
In logic, a manyvalued logic (also multi or multiplevalued logic) is a propositional calculus in which there are more than two truth values. Traditionally, in Aristotle's logical calculus, there were only two possible values (i.e., "true" and "false") for any proposition. Classical twovalued logic may be extended to nvalued logic for n greater than 2. Those most popular in the literature are threevalued (e.g., Łukasiewicz's and Kleene's, which accept the values "true", "false", and "unknown"), the finitevalued (finitelymany valued) with more than three values, and the infinitevalued (infinitelymany valued), such as fuzzy logic and probability logic.
History
The first known classical logician who didn't fully accept the law of excluded middle was Aristotle (who, ironically, is also generally considered to be the first classical logician and the "father of logic"^{[1]}). Aristotle admitted that his laws did not all apply to future events (De Interpretatione, ch. IX), but he didn't create a system of multivalued logic to explain this isolated remark. Until the coming of the 20th century, later logicians followed Aristotelian logic, which includes or assumes the law of the excluded middle.
The 20th century brought back the idea of multivalued logic. The Polish logician and philosopher Jan Łukasiewicz began to create systems of manyvalued logic in 1920, using a third value, "possible", to deal with Aristotle's paradox of the sea battle. Meanwhile, the American mathematician, Emil L. Post (1921), also introduced the formulation of additional truth degrees with n ≥ 2, where n are the truth values. Later, Jan Łukasiewicz and Alfred Tarski together formulated a logic on n truth values where n ≥ 2. In 1932 Hans Reichenbach formulated a logic of many truth values where n→infinity. Kurt Gödel in 1932 showed that intuitionistic logic is not a finitelymany valued logic, and defined a system of Gödel logics intermediate between classical and intuitionistic logic; such logics are known as intermediate logics.
Examples
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Kleene (strong) K_{3} and Priest logic P_{3}
Kleene's "(strong) logic of indeterminacy" K_{3} (sometimes ) and Priest's "logic of paradox" add a third "undefined" or "indeterminate" truth value I. The truth functions for negation (¬), conjunction (∧), disjunction (∨), implication (→_{K}), and biconditional (↔_{K}) are given by:^{[2]}





The difference between the two logics lies in how tautologies are defined. In K_{3} only T is a designated truth value, while in P_{3} both T and I are (a logical formula is considered a tautology if it evaluates to a designated truth value). In Kleene's logic I can be interpreted as being "underdetermined", being neither true nor false, while in Priest's logic I can be interpreted as being "overdetermined", being both true and false. K_{3} does not have any tautologies, while P_{3} has the same tautologies as classical twovalued logic.{{ safesubst:#invoke:Unsubstdate=__DATE__ $B= {{#invoke:Category handlermain}}{{#invoke:Category handlermain}}^{[citation needed]} }}
Bochvar's internal threevalued logic (also known as Kleene's weak threevalued logic)
Another logic is Bochvar's "internal" threevalued logic () also called Kleene's weak threevalued logic. Except for negation and biconditional, its truth tables are all different from the above.^{[3]}



The intermediate truth value in Bochvar's "internal" logic can be described as "contagious" because it propagates in a formula regardless of the value of any other variable.^{[4]}
Belnap logic (B_{4})
Belnap's logic B_{4} combines K_{3} and P_{3}. The overdetermined truth value is here denoted as B and the underdetermined truth value as N.



Semantics
Matrix semantics (logical matrices)
Proof theory
Relation to classical logic
Logics are usually systems intended to codify rules for preserving some semantic property of propositions across transformations. In classical logic, this property is "truth." In a valid argument, the truth of the derived proposition is guaranteed if the premises are jointly true, because the application of valid steps preserves the property. However, that property doesn't have to be that of "truth"; instead, it can be some other concept.
Multivalued logics are intended to preserve the property of designationhood (or being designated). Since there are more than two truth values, rules of inference may be intended to preserve more than just whichever corresponds (in the relevant sense) to truth. For example, in a threevalued logic, sometimes the two greatest truthvalues (when they are represented as e.g. positive integers) are designated and the rules of inference preserve these values. Precisely, a valid argument will be such that the value of the premises taken jointly will always be less than or equal to the conclusion.
For example, the preserved property could be justification, the foundational concept of intuitionistic logic. Thus, a proposition is not true or false; instead, it is justified or flawed. A key difference between justification and truth, in this case, is that the law of excluded middle doesn't hold: a proposition that is not flawed is not necessarily justified; instead, it's only not proven that it's flawed. The key difference is the determinacy of the preserved property: One may prove that P is justified, that P is flawed, or be unable to prove either. A valid argument preserves justification across transformations, so a proposition derived from justified propositions is still justified. However, there are proofs in classical logic that depend upon the law of excluded middle; since that law is not usable under this scheme, there are propositions that cannot be proven that way.
Suszko's thesis
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Applications
Known applications of manyvalued logic can be roughly classified into two groups.^{[5]} The first group uses manyvalued logic domain to solve binary problems more efficiently. For example, a wellknown approach to represent a multipleoutput Boolean function is to treat its output part as a single manyvalued variable and convert it to a singleoutput characteristic function. Other applications of manyvalued logic include design of Programmable Logic Arrays (PLAs) with input decoders, optimization of finite state machines, testing, and verification.
The second group targets the design of electronic circuits which employ more than two discrete levels of signals, such as manyvalued memories, arithmetic circuits, Field Programmable Gate Arrays (FPGA) etc. Manyvalued circuits have a number of theoretical advantages over standard binary circuits. For example, the interconnect on and off chip can be reduced if signals in the circuit assume four or more levels rather than only two. In memory design, storing two instead of one bit of information per memory cell doubles the density of the memory in the same die size. Applications using arithmetic circuits often benefit from using alternatives to binary number systems. For example, residue and redundant number systems can reduce or eliminate the ripplethrough carries which are involved in normal binary addition or subtraction, resulting in highspeed arithmetic operations. These number systems have a natural implementation using manyvalued circuits. However, the practicality of these potential advantages heavily depends on the availability of circuit realizations, which must be compatible or competitive with presentday standard technologies.
Research venues
An IEEE International Symposium on MultipleValued Logic (ISMVL) has been held annually since 1970. It mostly caters to applications in digital design and verification.^{[6]} There is also a Journal of MultipleValued Logic and Soft Computing.^{[7]}
See also
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 Mathematical logic
 Degrees of truth
 Fuzzy logic
 Gödel logic
 Kleene logic
 Kleene algebra (with involution)
 Łukasiewicz logic
 MValgebra
 Post logic
 Principle of bivalence
 A. N. Prior
 Relevance logic
 Philosophical logic
 Digital logic
 MVCML, multiplevalued currentmode logic
 IEEE 1164 a ninevalued standard for VHDL
 IEEE 1364 a fourvalued standard for Verilog
 Noisebased logic
Notes
References
 ↑ Hurley, Patrick. A Concise Introduction to Logic, 9th edition. (2006).
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 ↑ Dubrova, Elena (2002). MultipleValued Logic Synthesis and Optimization, in Hassoun S. and Sasao T., editors, Logic Synthesis and Verification, Kluwer Academic Publishers, pp. 89114
 ↑ http://www.informatik.unitrier.de/~ley/db/conf/ismvl/index.html
 ↑ http://www.oldcitypublishing.com/MVLSC/MVLSC.html
Further reading
General
 Béziau J.Y. (1997), What is manyvalued logic ? Proceedings of the 27th International Symposium on MultipleValued Logic, IEEE Computer Society, Los Alamitos, pp. 117–121.
 Malinowski, Gregorz, (2001), ManyValued Logics, in Goble, Lou, ed., The Blackwell Guide to Philosophical Logic. Blackwell.
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 Cignoli, R. L. O., D'Ottaviano, I, M. L., Mundici, D., (2000). Algebraic Foundations of Manyvalued Reasoning. Kluwer.
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 S. Gottwald, A Treatise on ManyValued Logics. Studies in Logic and Computation, vol. 9, Research Studies Press: Baldock, Hertfordshire, England, 2001.
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 Hájek P., (1998), Metamathematics of fuzzy logic. Kluwer. (Fuzzy logic understood as manyvalued logic sui generis.)
Specific
 Alexandre Zinoviev, Philosophical Problems of ManyValued Logic, D. Reidel Publishing Company, 169p., 1963.
 Prior A. 1957, Time and Modality. Oxford University Press, based on his 1956 John Locke lectures
 Goguen J.A. 1968/69, The logic of inexact concepts, Synthese, 19, 325–373.
 Chang C.C. and Keisler H. J. 1966. Continuous Model Theory, Princeton, Princeton University Press.
 Gerla G. 2001, Fuzzy logic: Mathematical Tools for Approximate Reasoning, Kluwer Academic Publishers, Dordrecht.
 Pavelka J. 1979, On fuzzy logic I: Manyvalued rules of inference, Zeitschr. f. math. Logik und Grundlagen d. Math., 25, 45–52.
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External links
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 Stanford Encyclopedia of Philosophy: "Truth Values"—by Yaroslav Shramko and Heinrich Wansing.
 IEEE Computer Society's Technical Committee on MultipleValued Logic
 Resources for ManyValued Logic by Reiner Hähnle, Chalmers University
 Manyvalued Logics W3 Server (archived)
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 Carlos Caleiro, Walter Carnielli, Marcelo E. Coniglio and João Marcos, Two's company: "The humbug of many logical values" in {{#invoke:citation/CS1citation
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