### Table of Contents

## Heyting algebras

Abbreviation: **HA**

### Definition

A \emph{Heyting algebra} is a structure $\mathbf{A}=\langle A,\vee ,0,\wedge ,1,\to \rangle $ such that

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

$\to$ gives the residual of $\wedge$: $x\wedge y\leq z\Longleftrightarrow y\leq x\to z$

### Definition

A \emph{Heyting algebra} is a FLew-algebra $\mathbf{A}=\langle A,\vee ,0,\wedge ,1,\cdot ,\to \rangle $ such that

$x\wedge y=x\cdot y$

##### Morphisms

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

$h(x\vee y)=h(x)\vee h(y)$, $h(0)=0$, $h(x\wedge y)=h(x)\wedge h(y)$, $h(1)=1$, $h(x\to y)=h(x)\to h(y)$

### Examples

Example 1: The open sets of any topological space $\mathbf X$ form a Heyting algebra under the operations of union $\cup$, empty set $\emptyset$, intersection $\cap$, whole space $X$, and the operation $U\to V=$ interior of $(X - U)\cup V$.

Example 2: Any frame can be expanded to a unique Heyting algebra by defining $x\to y = \bigvee\{z:x\wedge z\le y\}$.

### Basic results

Any finite distributive lattice is the reduct of a unique Heyting algebra. More generally the same result holds for any complete and completely distributive lattice.

A Heyting algebra is subdirectly irreducible if and only if it has a unique coatom.

### Properties

Classtype | variety |
---|---|

Equational theory | decidable |

Quasiequational theory | decidable |

First-order theory | undecidable |

Locally finite | no |

Residual size | unbounded |

Congruence distributive | yes |

Congruence modular | yes |

Congruence n-permutable | yes, $n=2$ |

Congruence e-regular | yes, $e=1$ |

Congruence uniform | no |

Congruence extension property | yes |

Definable principal congruences | yes |

Equationally def. pr. cong. | yes |

Amalgamation property | yes |

Strong amalgamation property | yes |

Epimorphisms are surjective | yes |

### Finite members

$\begin{array}{lr}
f(1)= &1

f(2)= &1

f(3)= &1

f(4)= &2

f(5)= &3

f(6)= &5

f(7)= &8

f(8)= &15

f(9)= &26

f(10)= &47

f(11)= &82

f(12)= &151

f(13)= &269

f(14)= &494

f(15)= &891

f(16)= &1639

f(17)= &2978

f(18)= &5483

f(19)= &10006

f(20)= &18428

\end{array}$

Values known up to size 49 ^{1)}

### Subclasses

### Superclasses

### References

^{1)}Marcel Ern\'e;, Jobst Heitzig and J\“urgen Reinhold,\emph{On the number of distributive lattices}, Electron. J. Combin., \textbf{9}2002,Research Paper 24, 23 pp. (electronic)MRreview