# Ramsey transfer to semi-retractions

## TL;DR

This paper introduces semi-retractions and semi-direct product structures to analyze the preservation of the Ramsey property in ordered relational structures, and characterizes NIP theories via generalized indiscernible sequences.

## Contribution

It defines semi-retractions and semi-direct product structures, proving the Ramsey property is preserved under semi-retractions, and characterizes NIP theories using these structures.

## Key findings

- Semi-retractions preserve the Ramsey property.
- Semi-direct product structures also have the Ramsey property.
- NIP theories are characterized by generalized indiscernible sequences indexed by semi-direct products.

## Abstract

We introduce the notion of a {\it semi-retraction}. Given two structures $\A$ and $\B$, $\A$ is a semi-retraction of $\B$ if there exist quantifier-free type respecting maps $f: \B \raw \A$ and $g: \A \raw \B$ such that $f \circ g$ is an embedding. We say that a structure has the Ramsey property if its age does. Given two locally finite ordered structures $\A$ and $\B$, if $\A$ is a semi-retraction of $\B$ and $\B$ has the Ramsey property, then $\A$ also has the Ramsey property. We introduce notation for what we call semi-direct product structures, after the group construction known to preserve the Ramsey property.~\cite{kpt05} We introduce the notion of a color-homogenizing map, and use this notion to give a finitary argument that the semi-direct product structure of ordered relational structures with the Ramsey property must also have the Ramsey property. Finally, we characterize NIP theories using a generalized indiscernible sequence indexed by a semi-direct product structure.