#### DOI

10.17077/etd.t84isylg

#### Document Type

Dissertation

#### Date of Degree

Summer 2015

#### Degree Name

PhD (Doctor of Philosophy)

#### Degree In

Mathematics

#### First Advisor

Miodrag Iovanov

#### First Committee Member

Victor Camillo

#### Second Committee Member

Frauke Bleher

#### Third Committee Member

Philip Kutzko

#### Fourth Committee Member

Yangbo Ye

#### Fifth Committee Member

Miodrag Iovanov

#### Abstract

The property of having a finite number of orbits under the regular action has been used to study properties of rings and algebras. For example, in ring theory, Yasuyuki Hirano was able to use this property to show that rings with finitely many orbits under the regular action can be decomposed as direct sum of uniserial rings and a finite ring. In this thesis, we study modules under the regular action. More precisely, if *R* is a unital ring and *M* is a left(right) *R*-module, we describe all modules *M* that have finitely many orbits under the regular action. Along the way, we give a (new) module theoretical proof to the theorem of Yasuyuki Hirano on the classification of rings with finitely many orbits under the regular action which was proven using using methods from ring theory. Our charaterization of modules with finitely many orbits under the regular action shows a connection between algebras with finitely many submodules and distributive modules. A particular algebra that is of interest to us is the incidence algebra of a finite poset.

Incidence algebras were originally introduced in the 1960's by Gian-Carlo Rota as a way to study combinatorial problems but it became apparent later on that such algebras were an interesting object to study in their own right. They include ring theoretical examples such as the product of copies of a ring *R* and the upper triangular matrices over *R*. Robert B. Feinberg in his work on incidence algebras developed an internal characterization of incidence algebras of lower finite quasi-ordered sets. For example, he showed that an associative unital complete topological algebra Λ over a field *K*, where *K* has the discrete topology, is isomorphic to an incidence algebra if and only if

1. Λ has a faithful unital left module *M* with a distributive lattice of submodules. Further, every finitely generated submodule of *M* is finite dimensional and Λ has the coarset topology such that its action on *M* is continuous in Λ, when *M* has the discrete topology.

2. For every maximal closed ideal *J*, Λ/*J* is isomorphic to *M _{n}(k)* for some integer

*n*.

3. For every closed ideal *J*, the center Λ/*J* is isomorphic to the direct product of copies of *k*.

This thesis investigates the deformations of incidence algebras and how such deformations relate to cohomology. We show that distributivity of projective indecomposable modules of algebras largely characterizes precisely those algebras which are deformations of incidence algebras

#### Public Abstract

This thesis studies mathematical objects called modules under certain algebraic action. We characterize those objects for nice underline algebraic object called rings. This generalizes the work done by Yasuyuli Hirano when studying the same algebraic action for the underline algebraic objects.

In the second part of the thesis, we investigate the deformations of an algebraic object called incidence algebra and characterize all algebraic objects that are deformations of incidence algebra.

#### Keywords

publicabstract

#### Pages

vi, 48 pages

#### Bibliography

Includes bibliographical references (pages 47-48).

#### Copyright

Copyright 2015 Gerard Diant Koffi

#### Recommended Citation

Koffi, Gerard Diant. "Modules and orbits of the regular action, and deformations of incidence algebras." PhD (Doctor of Philosophy) thesis, University of Iowa, 2015.

https://doi.org/10.17077/etd.t84isylg.