This is an old revision of the document!

## De Morgan algebras

Abbreviation: DeMA

### Definition

A De Morgan algebra is a structure $\mathbf{A}=\langle A,\vee ,0,\wedge ,1,\neg\rangle$ such that

$\langle A,\vee ,0,\wedge ,1\rangle$ is a bounded distributive lattice

$\neg$ is a De Morgan involution: $\neg( x\wedge y) =\neg x\vee \neg y$, $\neg\neg x=x$

Remark: It follows that $\neg ( x\vee y) =\neg x\wedge \neg y$, $\ \neg 1=0$ and $\neg 0=1$ (e.g. $\neg 1=\neg 1\vee 0=\neg 1\vee\neg\neg 0= \neg(1\wedge\neg 0)=\neg\neg 0=0$). Thus $\neg$ is a dual automorphism.

##### Morphisms

Let $\mathbf{A}$ and $\mathbf{B}$ be De Morgan algebras. A morphism from $\mathbf{A}$ to $\mathbf{B}$ is a function $h:A\rightarrow B$ that is a homomorphism:

$h(x\vee y)=h(x)\vee h(y)$, $h(\neg x)=\neg h(x)$

### Examples

Example 1: Let $\{0<a,b<1\}$ be the 4-element lattice with $a,b$ incomparable, and define $'$ by $0'=1,a'=a,b'=b$.

### Basic results

The algebra in Example 1 generates the variety of De Morgan algebras, see e.g. http://www.math.uic.edu/~kauffman/DeMorgan.pdf

### Properties

Classtype Variety Decidable Yes Yes Yes Yes 4

### Finite members

$\begin{array}{lr} f(1)= &1\\ f(2)= &1\\ f(3)= &1\\ f(4)= &3\\ f(5)= &1\\ f(6)= &4\\ f(7)= &2\\ f(8)= &9\\ f(9)= &5\\ f(10)= &14\\ \end{array}$