inzicht - Mathematics - # Morita Equivalence

Labelled Graphs and Morita Equivalence of Inverse Semigroups: Characterizing Morita Equivalence Using Labelled Graphs

Q: Can this method of using labelled graphs be extended to characterize Morita equivalence in more general algebraic structures beyond inverse semigroups?

This is a very interesting question that touches upon active research areas. While the paper focuses specifically on inverse semigroups, the concept of Morita equivalence is much broader, applicable to various algebraic structures like rings, categories, and even C*-algebras. Here's a breakdown of potential extensions and challenges: Rings: Morita equivalence for rings is well-established. Directly extending the labelled graph approach might be difficult. Rings generally lack the natural order structure present in inverse semigroups, which is crucial for defining the graph. However, exploring graph-based representations of other ring invariants (like posets of ideals) could be fruitful. C-algebras:* The paper hints at applications to C*-algebras, which are deeply connected to groupoids and inverse semigroups. Generalizing labelled graphs to capture Morita equivalence of broader classes of C*-algebras is an active research area. Difficulties arise from the analytic nature of C*-algebras, requiring more sophisticated graph-like structures (e.g., higher-rank graphs, k-graphs) to capture their complexity. Generalizations of Inverse Semigroups: Exploring similar graph-based invariants for structures like étale groupoids (which are closely related to inverse semigroups) is a natural direction. The challenge lies in finding suitable graph-theoretic counterparts to the algebraic properties defining Morita equivalence in these settings. In summary, extending the labelled graph approach to other structures requires carefully identifying analogous concepts and overcoming challenges posed by the specific algebraic properties of those structures.

Q: What are the limitations of using labelled graphs for characterizing Morita equivalence in cases where the inverse semigroups do not have finite intervals?

The paper explicitly assumes finite intervals in the inverse semigroups for constructing the labelled graph and proving the Morita equivalence result. This assumption plays a crucial role in the following ways: Construction of the Labelled Graph: The edges in the labelled graph are defined based on the immediate predecessors of idempotents within a D-class. Finite intervals guarantee that there are finitely many such predecessors, leading to a well-defined, finite graph for each D-class. Coherent Sets and Diamonds: The notion of a coherent set of idempotent representatives relies on the behavior of idempotents within intervals. Finite intervals simplify the conditions for a set to be coherent, making it easier to handle the complexities arising from "diamonds" (non-comparable idempotents within a D-class). Limitations Without Finite Intervals: Infinite Graphs: Without finite intervals, the labelled graph could become infinite, making it potentially unwieldy for characterizing Morita equivalence. Coherent Sets: Defining and working with coherent sets become significantly more complex without finite intervals. The conditions for coherence might need substantial modification, and it's unclear if a suitable generalization exists. Alternative Approaches: For inverse semigroups without finite intervals, alternative methods for characterizing Morita equivalence might be necessary. These could involve more abstract techniques from category theory or exploring different invariants that don't rely on the finite interval property.

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This research paper introduces a novel method for characterizing the Morita equivalence of inverse semigroups, particularly those containing "diamonds," using labelled graphs constructed from their idempotent D-class representatives.

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Bibliographic Information: Duah, Z., Du Preez, S., Milan, D., Ramamurthy, S., & Vega, L. (2024). Labelled Graphs as a Morita Equivalence Invariant of Inverse Semigroups. arXiv preprint arXiv:2411.09015v1.
Research Objective: The paper aims to determine whether labelled graphs can serve as a Morita equivalence invariant for inverse semigroups, particularly those that are not fully characterized by directed graphs due to the presence of "diamonds" (substructures making the directed graph representation insufficient).
Methodology: The authors develop a method for constructing a labelled graph from a combinatorial inverse semigroup with 0 that possesses a "coherent set" of idempotent D-class representatives and finite intervals. They then demonstrate that this inverse semigroup is Morita equivalent to the inverse semigroup of the constructed labelled graph.
Key Findings: The paper establishes that the labelled graph constructed using the authors' method is a Morita equivalence invariant for a broad class of inverse semigroups, including those containing diamonds. This finding is further applied to demonstrate that the labelled graph associated with the inverse hull of a Markov shift completely determines its Morita equivalence class among all other inverse hulls of Markov shifts.
Main Conclusions: The research successfully introduces labelled graphs as a powerful tool for characterizing Morita equivalence in inverse semigroups, extending previous work that relied solely on directed graphs. This has significant implications for the study of inverse semigroup C*-algebras, potentially enabling the development of new "geometric" classification results.
Significance: This work provides a novel approach to understanding the structure and classification of inverse semigroups, a fundamental concept in abstract algebra with connections to areas like C*-algebra theory and dynamical systems.
Limitations and Future Research: The paper primarily focuses on combinatorial inverse semigroups with specific properties. Further research could explore the applicability of labelled graphs to broader classes of inverse semigroups or investigate the development of "graph moves" analogous to those used in the study of graph C*-algebras.

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Labelled graphs as a Morita equivalence invariant of inverse semigroups

by Zachary Duah... om arxiv.org 11-15-2024

https://arxiv.org/pdf/2411.09015.pdf

Labelled graphs as a Morita equivalence invariant of inverse semigroups

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Can this method of using labelled graphs be extended to characterize Morita equivalence in more general algebraic structures beyond inverse semigroups?

This is a very interesting question that touches upon active research areas. While the paper focuses specifically on inverse semigroups, the concept of Morita equivalence is much broader, applicable to various algebraic structures like rings, categories, and even C*-algebras.
Here's a breakdown of potential extensions and challenges:

Rings:  Morita equivalence for rings is well-established.  Directly extending the labelled graph approach might be difficult.  Rings generally lack the natural order structure present in inverse semigroups, which is crucial for defining the graph. However, exploring graph-based representations of other ring invariants (like posets of ideals) could be fruitful.

C-algebras:*  The paper hints at applications to C*-algebras, which are deeply connected to groupoids and inverse semigroups.  Generalizing labelled graphs to capture Morita equivalence of broader classes of C*-algebras is an active research area.  Difficulties arise from the analytic nature of C*-algebras, requiring more sophisticated graph-like structures (e.g., higher-rank graphs, k-graphs) to capture their complexity.

Generalizations of Inverse Semigroups:  Exploring similar graph-based invariants for structures like  étale groupoids (which are closely related to inverse semigroups) is a natural direction. The challenge lies in finding suitable graph-theoretic counterparts to the algebraic properties defining Morita equivalence in these settings.
In summary, extending the labelled graph approach to other structures requires carefully identifying analogous concepts and overcoming challenges posed by the specific algebraic properties of those structures.

What are the limitations of using labelled graphs for characterizing Morita equivalence in cases where the inverse semigroups do not have finite intervals?

The paper explicitly assumes finite intervals in the inverse semigroups for constructing the labelled graph and proving the Morita equivalence result. This assumption plays a crucial role in the following ways:

Construction of the Labelled Graph: The edges in the labelled graph are defined based on the immediate predecessors of idempotents within a D-class.  Finite intervals guarantee that there are finitely many such predecessors, leading to a well-defined, finite graph for each D-class.

Coherent Sets and Diamonds: The notion of a coherent set of idempotent representatives relies on the behavior of idempotents within intervals.  Finite intervals simplify the conditions for a set to be coherent, making it easier to handle the complexities arising from "diamonds" (non-comparable idempotents within a D-class).
Limitations Without Finite Intervals:

Infinite Graphs: Without finite intervals, the labelled graph could become infinite, making it potentially unwieldy for characterizing Morita equivalence.

Coherent Sets:  Defining and working with coherent sets become significantly more complex without finite intervals. The conditions for coherence might need substantial modification, and it's unclear if a suitable generalization exists.

Alternative Approaches:  For inverse semigroups without finite intervals, alternative methods for characterizing Morita equivalence might be necessary. These could involve more abstract techniques from category theory or exploring different invariants that don't rely on the finite interval property.

How can the insights gained from this research be applied to develop new algorithms for efficiently determining the Morita equivalence of inverse semigroups represented by labelled graphs?

The paper's results open up possibilities for developing algorithms to determine the Morita equivalence of inverse semigroups, particularly those arising from labelled graphs. Here are some potential directions:

Labelled Graph Isomorphism: The paper establishes a strong connection between the structure of the labelled graph and the Morita equivalence class of the inverse semigroup. This suggests that algorithms for checking isomorphism of labelled graphs could be adapted to determine Morita equivalence. Efficient algorithms for graph isomorphism, while a complex problem in general, exist and could be specialized for this purpose.

Finite State Automata: The labelled graphs considered in the paper can be viewed as a form of finite-state automata.  Leveraging existing algorithms from automata theory, such as those for checking the equivalence of finite-state automata, could provide efficient ways to determine Morita equivalence.

Exploiting Coherent Sets: The properties of coherent sets can be exploited to simplify the comparison of labelled graphs. Algorithms could focus on efficiently finding and comparing coherent sets within the labelled graphs, potentially reducing the computational complexity of determining Morita equivalence.

Applications to Markov Shifts: The paper specifically highlights the complete invariance of the labelled graph for inverse hulls of Markov shifts. This result can be used to develop specialized algorithms for efficiently deciding Morita equivalence within this important class of inverse semigroups.
Challenges and Considerations:

Complexity:  Determining Morita equivalence, even with labelled graphs, can be computationally challenging.  The efficiency of algorithms will depend on factors like the size of the alphabet, the number of vertices and edges in the graph, and the specific properties of the coherent sets.

Generalizations: Extending these algorithms to handle inverse semigroups without finite intervals or to more general algebraic structures will require further research and algorithmic innovations.