# De Morgan algebra

In mathematics, a De Morgan algebra (named after Augustus De Morgan, a British mathematician and logician) is a structure A = (A, ∨, ∧, 0, 1, ¬) such that:

In a De Morgan algebra:

do not always hold (when they do, the algebra becomes a Boolean algebra).

Remark: It follows that ¬( x∨y) = ¬x∧¬y, ¬1 = 0 and ¬0 = 1 (e.g. ¬1 = ¬1∨0 = ¬1∨¬¬0 = ¬(1∧¬0) = ¬¬0 = 0). Thus ¬ is a dual automorphism.

De Morgan algebras were introduced by Grigore Moisil[1][2] around 1935.[2] although without the restriction of having a 0 and an 1.[3] They were then variously called quasi-boolean algebras in the Polish school, e.g. by Rasiowa and also distributive i-lattices by J. A. Kalman.[2] (i-lattice being an abbreviation for lattice with involution.) They have been further studied in the Argentian algebraic logic school of Antonio Monteiro.[1][2]

De Morgan algebras are important for the study of the mathematical aspects of fuzzy logic. The standard fuzzy algebra F = ([0,  1], max(xy), min(xy), 0, 1, 1 − x) is an example of a De Morgan algebra where the laws of excluded middle and noncontradiction do not hold.

Another example is Dunn's 4-valued logic, in which false < neither-true-nor-false < true and false < both-true-and-false < true, while neither-true-nor-false and both-true-and-false are not comparable.[2]

## Kleene algebra

If a De Morgan algebra additionally satisfies x ∧ ¬xy ∨ ¬y, it is called a Kleene algebra.[3][1] (This notion should not to be confused with the other Kleene algebra generalizing regular expressions.) This notion has also been called a normal i-lattice by J. A. Kalman.

Examples of Kleene algebras in the sense defined above include: lattice-ordered groups, Post algebras and Łukasiewicz algebras.[3] Boolean algebras also meet this definition of Kleene algebra. The simplest Kleene algebra that is not Boolean is Kleene's three-valued logic K3.[4] K3 made its first appearance in Kleene's On notation for ordinal numbers (1938).[5] The algebra was named after Kleene by Brignole and Monteiro.[6]

## Related notions

De Morgan algebra is not the only plausible way to generalize the Boolean algebra. Another way is to keep ¬x ∧ x = 0 (i.e. the law of noncontradiction) but to drop the law of the excluded middle. This approach (called semicomplementation) is well-defined even for a [meet] semilattice; if the set of semicomplements has a greatest element it is usually called pseudocomplement. If the pseudocomplement thus defined satisfies the law of the excluded middle, the resulting algebra is also Boolean. However, if only the weaker law ¬x ∨ ¬¬x = 1 is required, this results in Stone algebras.[1] More generally, both De Morgan and Stone algebras are proper subclasses of Ockham algebras.

## References

1. ^ a b c d Blyth, T. S.; Varlet, J. C. (1994). Ockham algebras. Oxford University Press. pp. 4–5. ISBN 978-0-19-859938-8.
2. Béziau, Jean-Yves (2012). "A History of Truth-Values". In Dov M. Gabbay, Francis Jeffry Pelletier, and John Woods. Logic: A History of its Central Concepts. North Holland (an imprint of Elsevier). pp. 280–281. ISBN 978-0-08-093170-8.
3. ^ a b c Cignoli, Roberto (1975). "Injective de Morgan and Kleene Algebras". Proceedings of the American Mathematical Society 47 (2): 269–278. doi:10.1090/S0002-9939-1975-0357259-4.
4. ^ Kalle Kaarli; Alden F. Pixley (21 July 2000). Polynomial Completeness in Algebraic Systems. CRC Press. pp. 297–. ISBN 978-1-58488-203-9.
5. ^ http://www.jstor.org/stable/2267778
6. ^ Brignole, D. and Monteiro, A. Caracterisation des algebres de Nelson par des egalites, Notas de Logica Matematica, Instituto de Matematica Universidad del sur Bahia Blanca 20 (1964) A (possibly abbreviated) version of this paper appeared later in Proc. Acad. Japan doi:10.3792/pja/1195521624 doi:10.3792/pja/1195521625