orthocomplemented lattice
An orthocomplemented lattice is a complemented lattice^{} in which every element has a distinguished complement, called an orthocomplement, that behaves like the complementary subspace of a subspace^{} in a vector space^{}.
Formally, let $L$ be a complemented lattice and denote $M$ the set of complements of elements of $L$. $M$ is clearly a subposet of $L$, with $\le $ inherited from $L$. For each $a\in L$, let ${M}_{a}\subseteq M$ be the set of complements of $a$. $L$ is said to be orthocomplemented if there is a function ${}^{\u27c2}:L\to M$, called an orthocomplementation, whose image is written ${a}^{\u27c2}$ for any $a\in L$, such that

1.
${a}^{\u27c2}\in {M}_{a}$,

2.
${({a}^{\u27c2})}^{\u27c2}=a$, and

3.
$\u27c2$ is orderreversing; that is, for any $a,b\in L$, $a\le b$ implies ${b}^{\u27c2}\le {a}^{\u27c2}$.
The element ${a}^{\u27c2}$ is called an orthocomplement of $a$ (via ${}^{\u27c2}$).
Examples. In addition to the example of the lattice^{} of vector subspaces of a vector space cited above, let’s look at the Hasse diagrams of the two finite complemented lattices below,
$$\text{xymatrix}\mathrm{\&}1\text{ar}\mathrm{@}[ld]\text{ar}\mathrm{@}[d]\text{ar}\mathrm{@}[rd]\mathrm{\&}a\text{ar}\mathrm{@}[rd]\mathrm{\&}b\text{ar}\mathrm{@}[d]\mathrm{\&}c\text{ar}\mathrm{@}[ld]\mathrm{\&}0\mathrm{\&}\mathit{\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.5em}\u2006}\text{xymatrix}\mathrm{\&}\mathrm{\&}1\text{ar}\mathrm{@}[lld]\text{ar}\mathrm{@}[ld]\text{ar}\mathrm{@}[rd]\text{ar}\mathrm{@}[rrd]\mathrm{\&}\mathrm{\&}a\text{ar}\mathrm{@}[rrd]\mathrm{\&}b\text{ar}\mathrm{@}[rd]\mathrm{\&}\mathrm{\&}c\text{ar}\mathrm{@}[ld]\mathrm{\&}d\text{ar}\mathrm{@}[lld]\mathrm{\&}\mathrm{\&}0\mathrm{\&}\mathrm{\&}$$ 
the one on the right is orthocomplemented, while the one on the left is not. From this one deduces that orthcomplementation is not unique, and that the cardinality of any finite orthocomplemented lattice is even.
Remarks.

•
From the first condition above, we see that an orthocomplementation $\u27c2$ is a bijection. It is onetoone: if ${a}^{\u27c2}={b}^{\u27c2}$, then $a={({a}^{\u27c2})}^{\u27c2}={({b}^{\u27c2})}^{\u27c2}=b$. And it is onto: if we pick $a\in M\subseteq L$, then ${({a}^{\u27c2})}^{\u27c2}=a$. As a result, $M=L$, every element of $L$ is an orthocomplement. Furthermore, we have ${0}^{\u27c2}=1$ and ${1}^{\u27c2}=0$.

•
Let ${L}^{\prime}$ be the dual lattice of $L$ (a lattice having the same underlying set, but with meet and join operations^{} switched). Then any orthocomplementation $\u27c2$ can be viewed as a lattice isomorphism^{} between $L$ and ${L}^{\prime}$.

•
From the above conditions, it follows that elements of $L$ satisfy the de Morgan’s laws: for $a,b\in L$, we have
${a}^{\u27c2}\wedge {b}^{\u27c2}={(a\vee b)}^{\u27c2},$ (1) ${a}^{\u27c2}\vee {b}^{\u27c2}={(a\wedge b)}^{\u27c2}.$ (2) To derive the first equation, first note $a\le a\vee b$. Then ${(a\vee b)}^{\u27c2}\le {a}^{\u27c2}$. Similarly, ${(a\vee b)}^{\u27c2}\le {b}^{\u27c2}$. So ${(a\vee b)}^{\u27c2}\le {a}^{\u27c2}\wedge {b}^{\u27c2}$. For the other inequality, we start with ${a}^{\u27c2}\wedge {b}^{\u27c2}\le {a}^{\u27c2}$. Then $a\le {({a}^{\u27c2}\wedge {b}^{\u27c2})}^{\u27c2}$. Similarly, $b\le {({a}^{\u27c2}\wedge {b}^{\u27c2})}^{\u27c2}$. Therefore, $a\vee b\le {({a}^{\u27c2}\wedge {b}^{\u27c2})}^{\u27c2}$, which implies that ${a}^{\u27c2}\wedge {b}^{\u27c2}\le {(a\vee b)}^{\u27c2}$.

•
Conversely, any of two equations in the previous remark can replace the third condition in the definition above. For example, suppose we have the second equation ${a}^{\u27c2}\vee {b}^{\u27c2}={(a\wedge b)}^{\u27c2}$. If $a\le b$, then $a=a\wedge b$, so ${a}^{\u27c2}={(a\wedge b)}^{\u27c2}={a}^{\u27c2}\vee {b}^{\u27c2}$, which shows that ${b}^{\u27c2}\le {a}^{\u27c2}$.

•
From the example above, one sees that orthocomplementation need not be unique. An orthocomplemented lattice with a unique orthocomplementation is said to be uniquely orthocomplemented. A uniquely complemented lattice^{} that is also orthocomplemented is uniquely orthocomplemented.

•
Orthocomplementation can be more generally defined over a bounded poset $P$ by requiring the orthocomplentation operator ${}^{\u27c2}$ to satisfy conditions 2 and 3 above, and a weaker version of condition 1: $a\wedge {a}^{\u27c2}$ exists and $=0$. Since ${}^{\u27c2}$ is an order reversing bijection on $P$, ${0}^{\u27c2}=1$ and ${1}^{\u27c2}=0$. From this, one deduces that $a\wedge {a}^{\u27c2}=0$ iff $a\vee {a}^{\u27c2}=1$. A bounded poset in which an orthocomplementation is defined is called an orthocomplemented poset.

•
In the category^{} of orthocomplemented lattices, the morphism^{} between a pair of objects is a $\{0,1\}$lattice homomorphism (http://planetmath.org/LatticeHomomorphism) $f$ that preserves orthocomplementation:
$$f({a}^{\u27c2})=f{(a)}^{\u27c2}.$$
References
 1 G. Birkhoff, Lattice Theory, AMS Colloquium Publications, Vol. XXV, 3rd Ed. (1967).
Title  orthocomplemented lattice 
Canonical name  OrthocomplementedLattice 
Date of creation  20130322 15:50:36 
Last modified on  20130322 15:50:36 
Owner  CWoo (3771) 
Last modified by  CWoo (3771) 
Numerical id  20 
Author  CWoo (3771) 
Entry type  Definition 
Classification  msc 03G12 
Classification  msc 06C15 
Synonym  ortholattice 
Synonym  uniquely orthocomplemented 
Related topic  ComplementedLattice 
Related topic  OrthomodularLattice 
Defines  orthocomplement 
Defines  orthocomplemented 
Defines  orthocomplementation 
Defines  orthocomplemented poset 
Defines  uniquely orthocomplemented lattice 