−Table of Contents
Ordered semilattices
Abbreviation: OSlat
Definition
An \emph{ordered semilattice} is a ordered semigroup A=⟨A,⋅,≤⟩A=⟨A,⋅,≤⟩ that is
\emph{commutative}: x⋅y=y⋅xx⋅y=y⋅x and
\emph{idempotent}: x⋅x=xx⋅x=x
Morphisms
Let AA and BB be ordered semigroups. A morphism from AA to BB is a function h:A→Bh:A→B that is a orderpreserving homomorphism: h(x⋅y)=h(x)⋅h(y)h(x⋅y)=h(x)⋅h(y), x≤y⟹h(x)≤h(y)x≤y⟹h(x)≤h(y).
Examples
Example 1:
Basic results
Properties
Finite members
f(1)=1f(2)=2f(3)=5f(4)=14f(5)=42f(6)=132f(7)=f(8)=
This sequence is the Catalan numbers http://oeis.org/A000108