# Embedding Semigroup $C^*$-algebras into Inductive Limits

## TL;DR

This paper explores how semigroup $C^*$-algebras, especially those related to the additive rationals, can be embedded into inductive limits constructed from Toeplitz algebras over partially ordered sets, revealing connections between topology and algebraic limits.

## Contribution

It demonstrates an embedding of the reduced semigroup $C^*$-algebra of the rationals into a specific inductive limit of Toeplitz algebra systems, linking topology and algebraic structures.

## Key findings

- Embedding of the rational semigroup $C^*$-algebra into an inductive limit.
- Connection established between topology on index sets and properties of inductive limits.
- Analysis of $C^*$-algebra systems over partially ordered sets.

## Abstract

The note is concerned with inductive systems of Toeplitz algebras and their $*$-homomorphisms over arbitrary partially ordered sets. The Toeplitz algebra is the reduced semigroup $C^*$-algebra for the additive semigroup of non-negative integers. It is known that every partially ordered set can be represented as the union of the family of its maximal upward directed subsets indexed by elements of some set. In our previous work we have studied a topology on this set of indexes. For every maximal upward directed subset we consider the inductive system of Toeplitz algebras that is defined by a given inductive system over an arbitrary partially ordered set and its inductive limit. Then for a base neighbourhood $U_a$ of the topology on the set of indexes we construct the $C^*$-algebra $\mathfrak{B}_a$ which is the direct product of those inductive limits. In this note we continue studying the connection between the properties of the topology on the set of indexes and properties of inductive limits for systems consisting of $C^*$-algebras $\mathfrak{B}_a$ and their $*$-homorphisms. It is proved that there exists an embedding of the reduced semigroup $C^*$-algebra for a semigroup in the additive group of all rational numbers into the inductive limit for the system of $C^*$-algebras $\mathfrak{B}_a$.