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Difference between revisions of "Semi-lattice"

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A commutative idempotent [[Semi-group|semi-group]], that is, a semi-group satisfying the identities <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s0842001.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s0842002.png" />. Every semi-lattice <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s0842003.png" /> can be turned into a [[Partially ordered set|partially ordered set]] (the partial order <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s0842004.png" /> is defined by the relation <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s0842005.png" /> if and only if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s0842006.png" />) in which for any pair of elements there is a least upper bound <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s0842007.png" />. Conversely, every partially ordered set with least upper bounds for every pair of elements is a semi-lattice with respect to the operation <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s0842008.png" />. In this case one says that the partially ordered set is an upper semi-lattice (or a join semi-lattice, or a <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s08420010.png" />-semi-lattice). A lower semi-lattice, also called a meet semi-lattice or a <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s084/s084200/s08420012.png" />-semi-lattice, is dually defined as a partially ordered set in which any two elements have a greatest lower bound.
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A commutative idempotent [[Semi-group|semi-group]], that is, a semi-group satisfying the identities $x+y=y+x$ and $x+x=x$. Every semi-lattice $p=\langle p,+\rangle$ can be turned into a [[Partially ordered set|partially ordered set]] (the partial order $\leq$ is defined by the relation $a\leq b$ if and only if $a+b=b$) in which for any pair of elements there is a least upper bound $\sup\{a,b\}=a+b$. Conversely, every partially ordered set with least upper bounds for every pair of elements is a semi-lattice with respect to the operation $a+b=\sup\{a,b\}$. In this case one says that the partially ordered set is an upper semi-lattice (or a join semi-lattice, or a $\vee$-semi-lattice). A lower semi-lattice, also called a meet semi-lattice or a $\wedge$-semi-lattice, is dually defined as a partially ordered set in which any two elements have a greatest lower bound.
  
  

Revision as of 09:38, 11 August 2014

A commutative idempotent semi-group, that is, a semi-group satisfying the identities $x+y=y+x$ and $x+x=x$. Every semi-lattice $p=\langle p,+\rangle$ can be turned into a partially ordered set (the partial order $\leq$ is defined by the relation $a\leq b$ if and only if $a+b=b$) in which for any pair of elements there is a least upper bound $\sup\{a,b\}=a+b$. Conversely, every partially ordered set with least upper bounds for every pair of elements is a semi-lattice with respect to the operation $a+b=\sup\{a,b\}$. In this case one says that the partially ordered set is an upper semi-lattice (or a join semi-lattice, or a $\vee$-semi-lattice). A lower semi-lattice, also called a meet semi-lattice or a $\wedge$-semi-lattice, is dually defined as a partially ordered set in which any two elements have a greatest lower bound.


Comments

A band is a semi-group every element of which is idempotent (cf. also Band of semi-groups) (which is a decomposition of a semi-group into sub-semi-groups forming a band). Thus, an upper (lower) semi-lattice defines a commutative band, and conversely.

References

[a1] A.H. Clifford, G.B. Preston, "The algebraic theory of semigroups" , 1 , Amer. Math. Soc. (1961) pp. §1.8
How to Cite This Entry:
Semi-lattice. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Semi-lattice&oldid=32831
This article was adapted from an original article by T.S. Fofanova (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article