# Cross-connections and variants of the full transformation semigroup

## TL;DR

This paper uses cross-connection theory to analyze the ideal structure of the regular part of a variant of the full transformation semigroup, providing a new structural and representational understanding.

## Contribution

It characterizes the ideal structure of the regular part of the variant semigroup using cross-connections and provides a structure theorem and representation as a cross-connection semigroup.

## Key findings

- Describes the ideal structure of $Reg( ext{T}_X^ heta)$ using cross-connections.
- Characterizes categories and their cross-connections induced by $ heta$.
- Provides a structure theorem and representation as a cross-connection semigroup.

## Abstract

Cross-connection theory propounded by K. S. S. Nambooripad describes the ideal structure of a regular semigroup using the categories of principal left (right) ideals. A variant $\mathscr{T}_X^\theta$ of the full transformation semigroup $(\mathscr{T}_X,\cdot)$ for an arbitrary $\theta \in \mathscr{T}_X$ is the semigroup $\mathscr{T}_X^\theta= (\mathscr{T}_X,\ast)$ with the binary operation $\alpha \ast \beta = \alpha\cdot\theta\cdot\beta$ where $\alpha, \beta \in \mathscr{T}_X$. In this article, we describe the ideal structure of the regular part $Reg(\mathscr{T}_X^\theta)$ of the variant of the full transformation semigroup using cross-connections. We characterize the constituent categories of $Reg(\mathscr{T}_X^\theta)$ and describe how they are \emph{cross-connected} by a functor induced by the sandwich transformation $\theta$. This lead us to a structure theorem for the semigroup and give the representation of $Reg(\mathscr{T}_X^\theta)$ as a cross-connection semigroup. Using this, we give a description of the biordered set and the sandwich sets of the semigroup.