# Skew lattice

In abstract algebra, a **skew lattice** is an algebraic structure that is a non-commutative generalization of a lattice. While the term *skew lattice* can be used to refer to any non-commutative generalization of a lattice, over the past twenty years it has been used primarily as follows.

## Definition

A **skew lattice** is a set *S* equipped with two associative, idempotent binary operations and , called *meet* and *join*, that satisfy the following dual pair of absorption laws

Given that and are associative and idempotent, these identities are equivalent to the dualities:

^{[1]}

## Historical background

For over 60 years, noncommutative variations of lattices have been studied with differing motivations. For some the motivation has been an interest in the conceptual boundaries of lattice theory; for others it was a search for noncommutative forms of logic and Boolean algebra; and for others it has been the behavior of idempotents in rings. A *noncommutative lattice*, generally speaking, is an algebra where and are associative, idempotent binary operations connected by absorption identities guaranteeing that in some way dualizes . The precise identities chosen depends upon the underlying motivation, with differing choices producing distinct varieties of algebras. *Pascual Jordan*, motivated by questions in quantum logic, initiated a study of *noncommutative lattices* in his 1949 paper, *Über Nichtkommutative Verbande*,^{[2]} choosing the absorption identities

He referred to those algebras satisfying them as *Schrägverbände*. By varying or augmenting these identities, Jordan and others obtained a number of varieties of noncommutative lattices.
Beginning with Jonathan Leech's 1989 paper, *Skew lattices in rings*,^{[3]} skew lattices as defined above have been the primary objects of study. This was aided by previous results about bands. This was especially the case for many of the basic properties.

## Basic properties

**Natural partial order and natural quasiorder**

In a skew lattice , the natural partial order is deﬁned by if , or dually, . The natural preorder on is given by if or dually . While and agree on lattices, properly reﬁnes in the noncommutative case. The induced natural equivalence is deﬁned by if , that is, and or dually, and . The blocks of the partition are lattice ordered by iﬀ and exist such that . This permits us to write Hasse diagrams of skew lattices such as the following pair:

E.g., in the diagram on the left above, that and are related is expressed by the dashed segment. The slanted lines reveal the natural partial order between elements of the distinct -classes. The elements , and form the singleton -classes.

**Rectangular Skew Lattices**

Skew lattices consisting of a single -class are called **rectangular**. They are characterized by the equivalent identities: , and . Rectangular skew lattices are isomorphic to skew lattices having the following construction (and conversely): given nonempty
sets and , on deﬁne and . The -class partition of a skew lattice , as indicated in the above diagrams, is the unique partition of into its maximal rectangular subalgebras, Moreover, is a congruence with the induced quotient algebra being the maximal lattice image of , thus making every skew lattice a lattice of rectangular subalgebras. This is the Cliﬀord-McLean Theorem for skew lattices, ﬁrst given for bands separately by Cliﬀord and McLean. It is also known as *the First Decomposition Theorem for skew lattices*.

**Right (left) handed skew lattices and the Kimura factorization**

A skew lattice is right-handed if it satisﬁes the identity or dually, . These identities essentially assert that and in each -class. Every skew lattice has a unique maximal right-handed image where the congruence is deﬁned by if both and (or dually, and ). Likewise a skew lattice is left-handed if and in each -class. Again the maximal left-handed image of a skew lattice is the image where the congruence is deﬁned in dual fashion to . Many examples of skew lattices are either right or left-handed. In the lattice of congruences, and is the identity congruence . The induced epimorphism factors through both induced epimorphisms and . Setting , the homomorphism deﬁned by , induces an isomorphism . This is the Kimura factorization of into a ﬁbred product of its maximal right and left-handed images.

Like the Cliﬀord-McLean Theorem, Kimura factorization (or the *Second Decomposition Theorem for skew lattices*) was ﬁrst given for regular bands (that satisfy the middle absorption
identity, ). Indeed both and are regular band operations. The above symbols , and come, of course, from basic semigroup theory.

For more details on the basic properties of a skew lattice please read ^{[3]}^{[4]}^{[5]}^{[6]}^{[7]}^{[8]}^{[9]} and.^{[10]}

## Subvarieties of skew lattices

Skew lattices form a variety. Rectangular skew lattices, left-handed and right-handed skew lattices all form subvarieties that are central to the basic structure theory of skew lattices. Here are several more.

**Symmetric Skew Lattices**

A skew lattice S is symmetric if for any , iﬀ . Occurrences of commutation are thus unambiguous for such skew lattices, with subsets of pairwise commuting elements generating commutative subalgebras, i.e, sublattices. ( This is not true for skew lattices in general.) Equational bases for this subvariety, ﬁrst given by Spinks ^{[11]} are:
and .
A **lattice section** of a skew lattice is a sublattice of meeting each -class of at a single element. is thus an internal copy of the lattice with the composition being an isomorphism. All symmetric skew lattices for which |S/D| \leq \aleph_0 , admit a lattice section.^{[10]} Symmetric or not, having a lattice section guarantees that also has internal copies of and given respectively by and , where and are the and congruence classes of in . Thus and are isomorphisms (See ^{[8]}). This leads to a commuting diagram of embedding dualizing the preceding Kimura diagram.

**Cancellative Skew Lattices**

A skew lattice is cancellative if and implies and likewise and implies . Cancellatice skew lattices are symmetric and can be shown to form a variety. Unlike lattices, they need not be distributive, and conversely.

**Distributive Skew Lattices**

Distributive skew lattices are determined by the identities: (D1 ) (D’1 )

Unlike lattices, (D1 ) and (D‘1 ) are not equivalent in general for skew lattices, but they are for symmetric skew lattices. (See,,.^{[9]}^{[12]}^{[13]}) The condition (D1 ) can be strengthened to (D2 ) in which case (D‘1 ) is a consequence. A skew lattice satisﬁes both (D2) and its dual, , if and only if it factors as the product of a distributive lattice and a rectangular skew lattice. In this latter case (D2 ) can be strengthened to and . (D3 )
On its own, (D3 ) is equivalent to (D2 ) when symmetry is added. (See.^{[3]}) We thus have six subvarieties of skew lattices determined respectively by (D1), (D2), (D3) and their duals.

**Normal Skew Lattices**

As seen above, and satisfy the identity . Bands satisfying the stronger identity, , are called normal. A skew lattice is normal skew if it satisﬁes

For each element a in a normal skew lattice , the set deﬁned by { } or equivalently {} is a sublattice of , and conversely. (Thus normal skew lattices have also been called local lattices.) When both and are normal, splits isomorphically into a product of a lattice and a rectangular skew lattice , and conversely. Thus both normal skew lattices and split skew lattices form varieties. Returning to distribution, so that characterizes the variety of distributive, normal skew lattices, and (D3) characterizes the variety of symmetric, distributive, normal skew lattices.

**Categorical Skew Lattices**

A skew lattice is categorical if nonempty composites of coset bijections are coset bijections. Categorical skew lattices form a variety. Skew lattices in rings and normal skew lattices are examples
of algebras on this variety.^{[4]} Let with , and , be the coset bijection from to taking to , be the coset bijection from to taking to and ﬁnally be the coset bijection from to taking to . A skew lattice is categorical if one always has the equality , ie. , if the
composite partial bijection if nonempty is a coset bijection from a -coset of to an -coset
of . That is .
All distributive skew lattices are categorical. Though symmetric skew lattices might not be. In a sense they reveal the independence between the properties of symmetry and distributivity.

For more details on these and other subvarieties of skew lattices please read ^{[3]}^{[6]}^{[9]}^{[10]}^{[11]}^{[13]}^{[14]} and.^{[4]}

## Skew Boolean algebras

A zero element in a skew lattice S is an element 0 of S such that for all , or, dually, . (0)

A Boolean skew lattice is a symmetric, distributive normal skew lattice with 0, , such that is a Boolean lattice for each . Given such skew lattice S , a diﬀerence operator \ is deﬁned on ^{[1]} by x\ y = where the latter is evaluated in the Boolean lattice . In the presence of (D3) and (0), \ is characterized by the identities:
and
One thus has a variety of skew Boolean algebras characterized by identities (D3), (0) and (S B). A primitive skew Boolean algebra consists of 0 and a single non-0 D-class. Thus it is the result of adjoining a 0 to a rectangular skew lattice D via (0) with , if
and otherwise. Every skew Boolean algebra is a subdirect product of primitive algebras. Skew Boolean algebras play an important role in the study of discriminator varieties and other generalizations in universal algebra of Boolean behavior. For more details on skew Boolean algebras see
^{[15]}
^{[16]}
^{[17]}
^{[18]}
^{[19]}
^{[20]}
^{[21]}
^{[22]}
^{[23]}
^{[24]}
.^{[25]}

## Skew lattices in rings

Let be a ring and let denote the set of all Idempotents in . For all set and .

Clearly but also is associative. If a subset is closed under and , then is a distributive, cancellative skew lattice. To ﬁnd such skew lattices in one looks at bands in , especially the ones that are maximal with respect to some constraint. In fact, every multiplicative band in that is maximal with respect to being right regular (= ) is also closed under and so forms a right-handed skew lattice. In general, every right regular band in generates a right-handed skew lattice in . Dual remarks also hold for left regular bands (bands satisfying the identity ) in . Maximal regular bands need not to be closed under as deﬁned; counterexamples are easily found using multiplicative rectangular bands. These cases are closed, however, under the cubic variant of deﬁned by since in these cases reduces to to give the dual rectangular band. By replacing the condition of regularity by normality , every maximal normal multiplicative band in is also closed under with , where , forms a Boolean skew lattice. When itself is closed under multiplication, then it is a normal band and thus forms a Boolean skew lattice. In fact, any skew Boolean algebra can be embedded into such an algebra. (See.^{[26]}) When A has a multiplicative identity , the condition that is multiplicatively closed is well-known to imply that forms a Boolean algebra. Skew lattices in rings continue to be a good source of examples and motivation. For more details read.^{[23]}^{[27]}^{[28]}^{[29]}^{[30]}

## Primitive skew lattices

Skew lattices consisting of exactly two D-classes are called primitive skew lattices. Given such a skew lattice with -classes in , then for any and , the subsets

are called, respectively, *cosets of A in B* and *cosets of B in A*. These cosets partition B and A with and . Cosets are always rectangular subalgebras in their -classes. What is more, the partial order induces a coset bijection defined by:

Collectively, coset bijections describe between the subsets and . They also determine and for pairs of elements from distinct -classes. Indeed, given and , let be the cost bijection between the cosets in and in . Then:

In general, given and with and , then belong to a common - coset in and belong to a common -coset in if and only if . Thus each coset bijection is, in some sense, a maximal collection of mutually parallel pairs .

Every primitive skew lattice factors as the fibred product of its maximal left and right- handed primitive images . Right-handed primitive skew lattices are constructed as follows. Let and be partitions of disjoint nonempty sets and , where all and share a common size. For each pair pick a fixed bijection from onto . On and separately set and ; but given and , set

where and with belonging to the cell of and belonging to the cell of . The various are the coset bijections. This is illustrated in the following partial Hasse diagram where and the arrows indicate the -outputs and from and .

One constructs left-handed primitive skew lattices in dual fashion. All right [left] handed primitive skew lattices can be constructed in this fashion. (See ^{[3]} Section 1.)

## The coset structure of skew lattices

A nonrectangular skew lattice is covered by its maximal primitive skew lattices: given comparable -classes in , forms a maximal primitive subalgebra of and every -class in lies in such a subalgebra. The coset structures on these primitive subalgebras combine to determine the outcomes and at least when and are comparable under . It turns out that and are determined in general by cosets and their bijections, although in
a slightly less direct manner than the -comparable case. In particular, given two incomparable D-classes A and B with join D-class J and meet D-class in , interesting connections arise between the two coset decompositions of J (or M) with respect to A and B. (See ^{[4]} Section 3.)

Thus a skew lattice may be viewed as a coset atlas of rectangular skew lattices placed on the vertices of a lattice and coset bijections between them, the latter seen as partial isomorphisms between the rectangular algebras with each coset bijection determining a corresponding pair of cosets. This perspective gives, in essence, the Hasse diagram of the skew lattice, which is easily drawn in cases of relatively small order. (See the diagrams in Section 3 above.) Given a chain of D-classes in , one has three sets of coset bijections: from A to B, from B to C and from A to C. In general, given coset bijections and , the composition of partial bijections could be empty. If it is not, then a unique coset bijection exists such that . (Again, is a bijection between a pair of cosets in and .) This inclusion can be strict. It is always an equality (given ) on a given skew lattice S precisely when S is categorical. In this case, by including the identity maps on each rectangular D-class and adjoining empty bijections between properly comparable D-classes, one has a category of rectangular algebras and coset bijections between them. The simple examples in Section 3 are categorical.

## See also

## References

- ↑
^{1.0}^{1.1}Leech, J, Skew lattices in rings, Algebra Universalis, 26(1989), 48-72. - ↑ Jordan, P. Uber Nichtkommutative Verbände, Arch. Math. 2 (1949), 56–59.
- ↑
^{3.0}^{3.1}^{3.2}^{3.3}^{3.4}Leech, J, Skew lattices in rings, Algebra Universalis, 26(1989), 48-72 - ↑
^{4.0}^{4.1}^{4.2}^{4.3}Leech, J, Recent developments in the theory of skew lattices, Semigroup Forum, 52(1996), 7-24. Cite error: Invalid`<ref>`

tag; name "LE96" defined multiple times with different content - ↑ Leech, J, Magic squares, ﬁnite planes and simple quasilattices, Ars Combinatoria 77(2005), 75-96.
- ↑
^{6.0}^{6.1}Leech, J, The geometry of skew lattices, Semigroup Forum, 52(1993), 7-24. - ↑ Leech, J, Normal skew lattices, Semigroup Forum, 44(1992), 1-8.
- ↑
^{8.0}^{8.1}Cvetko-Vah, K, Internal decompositions of skew lattices, Communications in Algebra, 35 (2007), 243-247 - ↑
^{9.0}^{9.1}^{9.2}Cvetko-Vah, K, A new proof of Spinks’ Theorem, Semigroup Forum 73 (2006), 267-272. - ↑
^{10.0}^{10.1}^{10.2}Laslo, G and Leech, J, Green’s relations on noncommutative lattices, Acta Sci. Math. (Szeged), 68 (2002), 501-533. - ↑
^{11.0}^{11.1}Spinks, M, Automated deduction in non-commutative lattice theory, Tech. Report 3/98, Monash U, GSCIT, 1998 - ↑ Spinks, M, Automated deduction in non-commutative lattice theory, Tech. Report 3/98, Monash University, Gippsland School of Computing and Information Technology, June 1998
- ↑
^{13.0}^{13.1}Spinks, M, On middle distributivity for skew lattices, Semigroup Forum 61 (2000). 341-345. - ↑ Cvetko-Vah, Karin ; Kinyon, M. ; Leech, J. ; Spinks, M. Skew Lattices with cancel lation. Pre-Print. Journal of Algebra and Its Applications, 2008
- ↑ Bignall, R. J., Quasiprimal Varieties and Components of Universal Algebras, Dissertation, The Flinders University of South Australia, 1976.
- ↑ Bignall, R J, A non-commutative multiple-valued logic, Proc. 21st International Symposium on Multiple-valued Logic, 1991, IEEE Computer Soc. Press, 49-54.
- ↑ Bignall, R J and J Leech, Skew Boolean algebras and discriminator varieties, Algebra Universalis, 33(1995), 387-398.
- ↑ Bignall, R J and M Spinks, Propositional skew Boolean logic, Proc. 26th International Symposium on Multiple-valued Logic, 1996, IEEE Computer Soc. Press, 43-48.
- ↑ Bignall, R J and M Spinks, Implicative BCS-algebra subreducts of skew Boolean algebras, Scientiae Mathematicae Japonicae, 58 (2003), 629-638.
- ↑ Bignall, R J and M Spinks, On binary discriminator varieties (I): Implicative BCS-algebras, International Journal of Algebra and Computation, to appear.
- ↑ Cornish, W H, Boolean skew algebras, Acta Math. Acad. Sci. Hung., 36 (1980), 281-291.
- ↑ Leech, J, Skew Boolean algebras, Algebra Universalis, 27(1990), 497-506.
- ↑
^{23.0}^{23.1}Leech and Spinks, Skew Boolean algebras generated from generalized Boolean algebras, Algebra Universalis 58 (2008), 287-302. Cite error: Invalid`<ref>`

tag; name "LE08" defined multiple times with different content - ↑ Spinks, M, Contributions to the Theory of Pre-BCK Algebras, Monash University Dissertation, 2002.
- ↑ Spinks, M and R Veroﬀ, Axiomatizing the skew Boolean propositional calculus, J.Automated Reasoning, 37 (2006), 3-20.
- ↑ Cvetko-Vah, K, Skew lattices in matrix rings, Algebra Universalis 53 (2005), 471-479.
- ↑ Cvetko-Vah, K, Pure skew lattices in rings, Semigroup Forum 68 (2004), 268-279.
- ↑ Cvetko-Vah, K, Pure ∇-bands, Semigroup Forum 71 (2005), 93-101.
- ↑ Cvetko-Vah, K, Skew lattices in rings, Dissertation, University of Ljubljana, 2005.
- ↑ Cvetko-Vah, K and J Leech, Associativity of the ∇-operation on bands in rings, Semigroup Forum 76 (2008), 32-50