### Table of Contents

## Action algebras

Abbreviation: **Act**

### Definition

An \emph{action algebra} is a structure $\mathbf{A}=\langle A,\vee,\bot,\cdot,1,^*,\backslash,/\rangle$ of type $\langle 2,0,2,0,1,2,2\rangle$ such that

$\langle A,\vee,\bot,\cdot,1,^*\rangle$ is a Kleene algebra

$\backslash $ is the left residual of $\cdot$: $y\leq x\backslash z\Longleftrightarrow xy\leq z$

$/$ is the right residual of $\cdot$: $x\leq z/y\Longleftrightarrow xy\leq z$

Remark: These equivalences can be written equationally.

##### Morphisms

Let $\mathbf{A}$ and $\mathbf{B}$ be action algebras. A morphism from $\mathbf{A}$ to $\mathbf{B}$ is a function $h:A\rightarrow B$ that is a homomorphism: $h(x\vee y)=h(x)\vee h(y)$, $h(x\cdot y)=h(x)\cdot h(y)$, $h(x\backslash y)=h(x)\backslash h(y)$, $h(x/y)=h(x)/h(y)$, $h(x^*)=h(x)^*$, $h(\bot)=\bot$ and $h(1)=1$.

### Examples

Example 1:

### Basic results

### Properties

Classtype | variety ^{1)} |
---|---|

Equational theory | |

Quasiequational theory | undecidable |

First-order theory | undecidable |

Locally finite | no |

Residual size | unbounded |

Congruence distributive | yes ^{2)} |

Congruence modular | yes |

Congruence n-permutable | yes, $n=4$ ^{3)} |

Congruence regular | |

Congruence uniform | |

Congruence extension property | |

Definable principal congruences | |

Equationally def. pr. cong. | |

Amalgamation property | |

Strong amalgamation property | |

Epimorphisms are surjective |

### Finite members

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

f(2)= &1

f(3)= &3

f(4)= &20

f(5)= &149

f(6)= &1488
\end{array}$

### Subclasses

### Superclasses

### References

^{1)}Vaughan Pratt, \emph{Action logic and pure induction}, ``Logics in AI (Amsterdam, 1990)'', Lecture Notes in Comput. Sci., 478, 1991, 97–120, 92d:03016

^{2), 3)}C.J. van Alten and J.G. Raftery, \emph{Embedding Theorems and Rule Separation in Logics without Weakening}, Studia Logica, 2004, …–…, preprint