Blow-ups and Extensions of Oriented Trees in Tournaments: Proving Linear Unavoidability

מושגי ליבה

This research paper demonstrates that the families of k-blow-ups and k-extensions of oriented trees are linearly unavoidable, meaning they can always be found within a tournament of a certain size relative to the tree's order.

תקציר

• Bibliographic Information: Aboulker, P., Havet, F., Lochet, W., Lopes, R., Picasarri-Arrieta, L., & Rambaud, C. (2024). Blow-ups and extensions of trees in tournaments. arXiv preprint arXiv:2410.23566.

• Research Objective: This paper investigates the unavoidability of acyclic digraphs, particularly focusing on k-blow-ups and k-extensions of oriented trees, within tournaments. The authors aim to determine if these families of digraphs are linearly unavoidable.

• Methodology: The research utilizes proof by induction and probabilistic methods, drawing upon concepts like median orders of tournaments, Kővári-Sós-Turán-like lemmas, and properties of transitive tournaments. The authors establish bounds on the minimum number of copies of specific subgraphs within tournaments to prove their results.

• Key Findings: The paper presents two main findings:

  • The k-blow-up of an oriented tree of order n is (2^(10+18k) * kn)-unavoidable.
  • Every k-extension of a sufficiently large oriented tree F is (2 * 3^(2k+2 choose 2) * |V(F)|)-unavoidable.

• Main Conclusions: The authors conclude that both k-blow-ups and k-extensions of oriented trees are linearly unavoidable. These results contribute to the understanding of the structural properties of tournaments and the embedding of acyclic digraphs within them.

• Significance: This research extends previous work on the unavoidability of oriented paths and trees in tournaments. It provides new insights into the family of linearly unavoidable digraphs and offers potential applications in areas like Ramsey theory and extremal graph theory.

• Limitations and Future Research: The provided bounds for the unavoidability of k-blow-ups and k-extensions of oriented trees are not claimed to be tight. Future research could focus on improving these bounds and exploring the unavoidability of other families of acyclic digraphs. Additionally, investigating the computational complexity of finding these unavoidable structures within tournaments could be of interest.

סטטיסטיקה

Every tournament of order N contains at least 1/8 * N * (N-1) * (N-3) copies of T3 (transitive tournament of order 3).
The number of distinct copies of Tk (transitive tournament of order k) in a tournament on N vertices (N >= 3*(k+1 choose 2)) is at least 3^-(k+1 choose 2) * N^k.

ציטוטים

תובנות מפתח מזוקקות מ:

Blow-ups and extensions of trees in tournaments

by Pier... ב- arxiv.org 11-01-2024

https://arxiv.org/pdf/2410.23566.pdf

שאלות מעמיקות

Can the techniques used in this paper be extended to prove the linear unavoidability of other families of digraphs beyond k-blow-ups and k-extensions of oriented trees?

This is a very interesting and open-ended question. Here's a breakdown of potential avenues and challenges:
Potential Extensions:

Digraphs with Bounded Pathwidth/Treewidth: The paper heavily relies on the tree-like structure of oriented trees. A natural extension would be to explore digraphs with bounded pathwidth or treewidth. These digraphs can be decomposed into parts arranged in a tree-like fashion, potentially allowing for adaptations of the inductive arguments used for trees.
Sparse Digraphs with Specific Properties: Instead of general bounded maximum average degree, focusing on digraphs with additional structural constraints (e.g., bounded circumference, bounded feedback vertex set) might be fruitful. The key would be to leverage these properties to control the spread of embeddings during the inductive steps.
Random Acyclic Digraphs: Analyzing the linear unavoidability of random acyclic digraphs with certain edge probability functions could be insightful. Techniques from probabilistic combinatorics, such as the first and second moment methods, would likely play a crucial role.
Challenges:

Loss of Median Order Properties: The median order arguments used for tournaments are powerful but rely on the specific structure of tournaments. Generalizing these arguments to arbitrary digraphs or even specific families might be challenging.
Finding Suitable Dense Substructures: The proofs rely on finding dense substructures (like the T T2k+1) in tournaments. Identifying analogous structures in other digraph families and relating their density to the unavoidability of the target digraphs would be crucial.
Handling Cycles: The acyclic nature of the considered digraphs is essential. Extending the results to digraphs with cycles would require fundamentally different approaches, as cycles introduce significant complexities in embedding problems.

What if we consider tournaments with specific properties, such as regularity or quasi-randomness, would the bounds for the unavoidability of these structures change?

Yes, considering tournaments with specific properties could lead to improved bounds or even exact results for the unavoidability of certain structures.
Regular or Quasi-Random Tournaments:

Higher Density of Substructures: Regular and quasi-random tournaments tend to have a more uniform distribution of edges, leading to a higher density of small substructures compared to general tournaments. This could potentially improve the lower bounds on the number of embeddings used in the proofs, leading to better unavoidability bounds.
Stronger Structural Properties: These tournaments often exhibit stronger structural properties that could be exploited. For instance, expansion properties in quasi-random tournaments might guarantee the existence of large sets of vertices with suitable connections, simplifying the embedding process.
Potential Improvements:

Tighter Bounds: The current bounds, particularly for k-blow-ups, are likely not tight. Exploiting the regularity or quasi-randomness could lead to tighter, possibly even optimal, bounds for the unavoidability of these structures.
Exact Results: In some cases, it might be possible to obtain exact unavoidability results for specific structures within these special tournaments. For example, the unavoidability of small oriented trees might be precisely determined in sufficiently large regular tournaments.
Example:
Consider the directed path ⃗Pn. In a random tournament, the probability of a given n-vertex subdigraph being ⃗Pn is exponentially small. However, in a regular tournament, one might be able to leverage the regularity to construct embeddings of ⃗Pn more easily, potentially leading to a smaller unavoidability threshold compared to general tournaments.

How can the concept of linear unavoidability in tournaments be applied to real-world scenarios, such as network design or resource allocation problems?

While the concept of linear unavoidability in tournaments might seem abstract, it has potential applications in various real-world scenarios by modeling relationships and dependencies:
Network Design:

Robust Network Topologies: In communication or transportation networks, linear unavoidability can guide the design of robust topologies. Ensuring the presence of certain unavoidable substructures, even with limited knowledge of the overall network, can guarantee certain connectivity or flow properties, making the network resilient to failures or disruptions.
Efficient Routing Protocols: Unavoidable structures can inform the design of efficient routing protocols. Knowing that certain patterns are inherently present in the network allows for the development of routing strategies that exploit these patterns, potentially reducing communication overhead or latency.
Resource Allocation:

Fair Scheduling: In scheduling problems where tasks have precedence constraints represented by a directed acyclic graph, linear unavoidability can help ensure fairness. If certain substructures representing fair allocations of resources are unavoidable, then any feasible schedule will inherently exhibit a degree of fairness.
Deadlock Avoidance: In distributed systems, understanding unavoidable patterns in the dependency graph of processes competing for resources can aid in designing deadlock avoidance mechanisms. By identifying and preventing the formation of unavoidable structures associated with deadlocks, the system can ensure progress.
Social Network Analysis:

Influence Propagation: Linear unavoidability can be relevant in studying influence propagation in social networks. Identifying unavoidable substructures in the network of relationships can help understand how information or opinions spread and might inform targeted advertising or intervention strategies.
Challenges and Considerations:

Model Simplification: Real-world networks are often more complex than tournaments. Applying the concept of linear unavoidability requires careful abstraction and simplification of the problem to a suitable tournament model.
Computational Complexity: Determining the unavoidability of structures, especially in large networks, can be computationally challenging. Efficient algorithms or approximation techniques might be necessary for practical applications.
Overall, while the connection between linear unavoidability and real-world problems might not always be direct, the concept provides a theoretical framework for understanding and potentially exploiting inherent structural properties in various domains.