arXiv subjects

Subrahmanyam Kalyanasundaram

Publications and source records attributed to Subrahmanyam Kalyanasundaram.

At least 19 recordsLinked to original sources

arXiv2026

Conflict-Free Cuts in Planar and 3-Degenerate Graphs with 1-Regular Conflicts

A conflict-free cut $F$ on a simple connected graph $G = (V, E)$ is defined as a set of edges $F \subseteq E$ such that $G-F$ is disconnected, and no two edges in $F$ are conflicting. The notion of conflicting edges is represented using an associated conflict graph $\widehat{G} = (\widehat{V}, \widehat{E})$ where $\widehat{V} = E$. Deciding if a given planar graph $G$, with an associated conflict graph $\widehat{G}$, has a conflict-free cut is known to be NP-complete, when $G$ has maximum degree four and $\widehat{G}$ is a line graph of $G$ [Bonsma, JGT 2009]. In this paper, we prove the following for the case when $\widehat{G}$ is 1-regular. * We completely resolve the complexity of the decision problem when $G$ is planar. Towards this end, we show that (a) there always exists a conflict-free cut when the graph is planar and 4-regular unless it is the octahedron graph and (b) the decision problem is NP-complete, even in the case when $G$ is planar with maximum degree 5. * We also show that the decision problem is NP-complete when $G$ is a 3-degenerate graph with maximum degree 5. This completely resolves the complexity status of the problem when $G$ is 3-degenerate. * We construct families of graphs with 1-regular conflict graphs that do not have a conflict-free cut. Our results answer the questions posed in [Rauch, Rautenbach and Souza, IPL 2025].

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arXiv2025

Extremal Results on Conflict-free Coloring

A conflict-free open neighborhood coloring of a graph is an assignment of colors to the vertices such that for every vertex there is a color that appears exactly once in its open neighborhood. For a graph $G$, the smallest number of colors required for such a coloring is called the conflict-free open neighborhood (CFON) chromatic number and is denoted by $χ_{ON}(G)$. By considering closed neighborhood instead of open neighborhood, we obtain the analogous notions of conflict-free closed neighborhood (CFCN) coloring, and CFCN chromatic number (denoted by $χ_{CN}(G)$). The notion of conflict-free coloring was introduced in 2002, and has since received considerable attention. In this paper, we study some extremal questions related to CFON and CFCN coloring.

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arXiv2024

A bound for the cops and robber problem in terms of 2-component order connectivity

In the cops and robber game, there are multiple cops and a single robber taking turns moving along the edges of a graph. The goal of the cops is to capture the robber (move to the same vertex as the robber) and the goal of the robber is to avoid capture. The cop number of a given graph is the smallest number of cops required to ensure the capture of the robber. The k-component order connectivity of a graph G = (V, E) is the size of a smallest set U, such that all the connected components of the induced graph on V \ U are of size at most k. In this brief note, we provide a bound on the cop number of graphs in terms of their 2-component order connectivity.

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arXiv2024

The Complexity of Optimizing Atomic Congestion

Atomic congestion games are a classic topic in network design, routing, and algorithmic game theory, and are capable of modeling congestion and flow optimization tasks in various application areas. While both the price of anarchy for such games as well as the computational complexity of computing their Nash equilibria are by now well-understood, the computational complexity of computing a system-optimal set of strategies -- that is, a centrally planned routing that minimizes the average cost of agents -- is severely understudied in the literature. We close this gap by identifying the exact boundaries of tractability for the problem through the lens of the parameterized complexity paradigm. After showing that the problem remains highly intractable even on extremely simple networks, we obtain a set of results which demonstrate that the structural parameters which control the computational (in)tractability of the problem are not vertex-separator based in nature (such as, e.g., treewidth), but rather based on edge separators. We conclude by extending our analysis towards the (even more challenging) min-max variant of the problem.

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arXiv2024

VEXIR2Vec: An Architecture-Neutral Embedding Framework for Binary Similarity

Binary similarity involves determining whether two binary programs exhibit similar functionality, often originating from the same source code. In this work, we propose VexIR2Vec, an approach for binary similarity using VEX-IR, an architecture-neutral Intermediate Representation (IR). We extract the embeddings from sequences of basic blocks, termed peepholes, derived by random walks on the control-flow graph. The peepholes are normalized using transformations inspired by compiler optimizations. The VEX-IR Normalization Engine mitigates, with these transformations, the architectural and compiler-induced variations in binaries while exposing semantic similarities. We then learn the vocabulary of representations at the entity level of the IR using the knowledge graph embedding techniques in an unsupervised manner. This vocabulary is used to derive function embeddings for similarity assessment using VexNet, a feed-forward Siamese network designed to position similar functions closely and separate dissimilar ones in an n-dimensional space. This approach is amenable for both diffing and searching tasks, ensuring robustness against Out-Of-Vocabulary (OOV) issues. We evaluate VexIR2Vec on a dataset comprising 2.7M functions and 15.5K binaries from 7 projects compiled across 12 compilers targeting x86 and ARM architectures. In diffing experiments, VexIR2Vec outperforms the nearest baselines by $40\%$, $18\%$, $21\%$, and $60\%$ in cross-optimization, cross-compilation, cross-architecture, and obfuscation settings, respectively. In the searching experiment, VexIR2Vec achieves a mean average precision of $0.76$, outperforming the nearest baseline by $46\%$. Our framework is highly scalable and is built as a lightweight, multi-threaded, parallel library using only open-source tools. VexIR2Vec is $3.1$-$3.5 \times$ faster than the closest baselines and orders-of-magnitude faster than other tools.

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arXiv2024

Conflict-Free Coloring: Graphs of Bounded Clique Width and Intersection Graphs

A conflict-free coloring of a graph $G$ is a (partial) coloring of its vertices such that every vertex $u$ has a neighbor whose assigned color is unique in the neighborhood of $u$. There are two variants of this coloring, one defined using the open neighborhood and one using the closed neighborhood. For both variants, we study the problem of deciding whether the conflict-free coloring of a given graph $G$ is at most a given number $k$. In this work, we investigate the relation of clique-width and minimum number of colors needed (for both variants) and show that these parameters do not bound one another. Moreover, we consider specific graph classes, particularly graphs of bounded clique-width and types of intersection graphs, such as distance hereditary graphs, interval graphs and unit square and disk graphs. We also consider Kneser graphs and split graphs. We give (often tight) upper and lower bounds and determine the complexity of the decision problem on these graph classes, which improve some of the results from the literature. Particularly, we settle the number of colors needed for an interval graph to be conflict-free colored under the open neighborhood model, which was posed as an open problem.

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arXiv2022

Pliable Index Coding via Conflict-Free Colorings of Hypergraphs

In the pliable index coding (PICOD) problem, a server is to serve multiple clients, each of which possesses a unique subset of the complete message set as side information and requests a new message which it does not have. The goal of the server is to do this using as few transmissions as possible. This work presents a hypergraph coloring approach to the scalar PICOD problem. A \textit{conflict-free coloring} of a hypergraph is known from literature as an assignment of colors to its vertices so that each hyperedge of the graph contains one uniquely colored vertex. For a given PICOD problem represented by a hypergraph consisting of messages as vertices and request-sets as hyperedges, we present achievable PICOD schemes using conflict-free colorings of the PICOD hypergraph. Various graph theoretic parameters arising out of such colorings (and some new coloring variants) then give a number of upper bounds on the optimal PICOD length, which we study in this work. Suppose the PICOD hypergraph has $m$ vertices and $n$ hyperedges, where every hyperedge overlaps with at most $Γ$ other hyperedges. We show easy to implement randomized algorithms for the following: (a) For the single request case, we give a PICOD of length $O(\log^2Γ)$. This result improves over known achievability results for some parameter ranges, (b) For the $t$-request case, we give an MDS code of length $\max(O(\log Γ\log m), O(t \log m))$. Further if the hyperedges (request sets) are sufficiently large, we give a PICOD of the same length as above, which is not based on MDS construction. In general, this gives an improvement over prior achievability results. Our codes are of near-optimal length (up to a multiplicative factor of $\log t$).

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arXiv2021

Conflict-free coloring on open neighborhoods of claw-free graphs

The `Conflict-Free Open (Closed) Neighborhood coloring', abbreviated CFON (CFCN) coloring, of a graph $G$ using $r$ colors is a coloring of the vertices of $G$ such that every vertex sees some color exactly once in its open (closed) neighborhood. The minimum $r$ such that $G$ has a CFON (CFCN) coloring using $r$ colors is called the `CFON chromatic number' (`CFCN chromatic number') of $G$. This is denoted by $χ_{CF}^{ON}(G)$ ($χ_{CF}^{CN}(G)$). D\k ebski and Przybyło in [J. Graph Theory, 2021] showed that if $G$ is a line graph with maximum degree $Δ$, then $χ_{CF}^{CN}(G) = O(\ln Δ)$. As an open question, they asked if the result could be extended to claw-free ($K_{1,3}$-free) graphs, which are a superclass of line graphs. For $k\geq 3$, we show that if $G$ is $K_{1,k}$-free, then $χ_{CF}^{ON}(G) = O(k^2\ln Δ)$. Since it is known that the CFCN chromatic number of a graph is at most twice its CFON chromatic number, this answers the question posed by Dębski and Przybyło.

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arXiv2020

A Tight Bound for Conflict-free Coloring in terms of Distance to Cluster

Given an undirected graph $G = (V,E)$, a conflict-free coloring with respect to open neighborhoods (CFON coloring) is a vertex coloring such that every vertex has a uniquely colored vertex in its open neighborhood. The minimum number of colors required for such a coloring is the CFON chromatic number of $G$, denoted by $χ_{ON}(G)$. In previous work [WG 2020], we showed the upper bound $χ_{ON}(G) \leq dc(G) + 3$, where $dc(G)$ denotes the distance to cluster parameter of $G$. In this paper, we obtain the improved upper bound of $χ_{ON}(G) \leq dc(G) + 1$. We also exhibit a family of graphs for which $χ_{ON}(G) > dc(G)$, thereby demonstrating that our upper bound is tight.

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arXiv2020

Conflict-Free Coloring of Star-Free Graphs on Open Neighborhoods

Given a graph, the conflict-free coloring problem on open neighborhoods (CFON) asks to color the vertices of the graph so that all the vertices have a uniquely colored vertex in its open neighborhood. The smallest number of colors required for such a coloring is called the conflict-free chromatic number and denoted $χ_{ON}(G)$. In this note, we study this problem on $S_k$-free graphs where $S_k$ is a star on $k+1$ vertices. When $G$ is $S_k$-free, we show that $χ_{ON}(G) = O(k\cdot \log^{2+ε}Δ)$, for any $ε> 0$, where $Δ$ denotes the maximum degree of $G$. Further, we show existence of claw-free ($S_3$-free) graphs that require $Ω(\log Δ)$ colors.

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arXiv2020

Combinatorial Bounds for Conflict-free Coloring on Open Neighborhoods

In an undirected graph $G$, a conflict-free coloring with respect to open neighborhoods (denoted by CFON coloring) is an assignment of colors to the vertices such that every vertex has a uniquely colored vertex in its open neighborhood. The minimum number of colors required for a CFON coloring of $G$ is the CFON chromatic number of $G$, denoted by $χ_{ON}(G)$. The decision problem that asks whether $χ_{ON}(G) \leq k$ is NP-complete. We obtain the following results: * Bodlaender, Kolay and Pieterse [WADS 2019] showed the upper bound $χ_{ON}(G)\leq {\sf fvs}(G)+3$, where ${\sf fvs}(G)$ denotes the size of a minimum feedback vertex set of $G$. We show the improved bound of $χ_{ON}(G)\leq {\sf fvs}(G)+2$, which is tight, thereby answering an open question in the above paper. * We study the relation between $χ_{ON}(G)$ and the pathwidth of the graph $G$, denoted ${\sf pw}(G)$. The above paper from WADS 2019 showed the upper bound $χ_{ON}(G) \leq 2{\sf tw}(G)+1$ where ${\sf tw}(G)$ stands for the treewidth of $G$. This implies an upper bound of $χ_{ON}(G) \leq 2{\sf pw}(G)+1$. We show an improved bound of $χ_{ON}(G) \leq \lfloor \frac{5}{3}({\sf pw}(G)+1) \rfloor$. * We prove new bounds for $χ_{ON}(G)$ with respect to the structural parameters neighborhood diversity and distance to cluster, improving existing results. * We also study the partial coloring variant of the CFON coloring problem, which allows vertices to be left uncolored. Let $χ^*_{ON}(G)$ denote the minimum number of colors required to color $G$ as per this variant. Abel et. al. [SIDMA 2018] showed that $χ^*_{ON}(G) \leq 8$ when $G$ is planar. They asked if fewer colors would suffice for planar graphs. We answer this question by showing that $χ^*_{ON}(G) \leq 5$ for all planar $G$. All our bounds are a result of constructive algorithmic procedures.

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arXiv2020

Conflict-free coloring on closed neighborhoods of bounded degree graphs

The closed neighborhood conflict-free chromatic number of a graph $G$, denoted by $χ_{CN}(G)$, is the minimum number of colors required to color the vertices of $G$ such that for every vertex, there is a color that appears exactly once in its closed neighborhood. Pach and Tardos [Combin. Probab. Comput. 2009] showed that $χ_{CN}(G) = O(\log^{2+\varepsilon} Δ)$, for any $\varepsilon > 0$, where $Δ$ is the maximum degree. In [Combin. Probab. Comput. 2014], Glebov, Szabó and Tardos showed existence of graphs $G$ with $χ_{CN}(G) = Ω(\log^2Δ)$. In this paper, we bridge the gap between the two bounds by showing that $χ_{CN}(G) = O(\log^2 Δ)$.

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arXiv2019

Conflict-Free Coloring on Open Neighborhoods

In an undirected graph, a conflict-free coloring (with respect to open neighborhoods) is an assignment of colors to the vertices of the graph $G$ such that every vertex in $G$ has a uniquely colored vertex in its open neighborhood. The conflict-free coloring problem asks to find the smallest number of colors required for a conflict-free coloring. The conflict-free coloring problem is NP-complete. From results in Abel et. al. [SODA 2017], it can be inferred that every planar graph has a conflict-free coloring with at most nine colors. As the best known lower bound for planar graphs is four colors, it was asked in the same paper if fewer colors would suffice. We make progress in answering this question, by showing that every planar graph can be colored using at most six colors. The same proof idea is used to show that every outerplanar graph can be colored using at most five colors. Using a different approach, we further show that every outerplanar graph can be colored using at most four colors. Finally, we study the problem on Kneser graphs. We show that $k+2$ colors are necessary and sufficient to color the Kneser graph $K(n,k)$ when $n\geq k(k+1)^2 + 1$.

cs.DM↗

arXiv2018

On the Tractability of (k,i)-Coloring

In an undirected graph, a proper (k,i)-coloring is an assignment of a set of k colors to each vertex such that any two adjacent vertices have at most i common colors. The (k,i)-coloring problem is to compute the minimum number of colors required for a proper (k,i)-coloring. This is a generalization of the classic graph coloring problem. We show a parameterized algorithm for the (k,i)-coloring problem with the size of the feedback vertex set as a parameter. Our algorithm does not use tree-width machinery, thus answering a question of Majumdar, Neogi, Raman and Tale [CALDAM 2017]. We also give a faster and simpler exact algorithm for (k, k-1)-coloring. From the hardness perspective, we show that the (k,i)-coloring problem is NP-complete for any fixed values i, k, whenever i<k, thereby settling a conjecture of Mendez-Diaz and Zabala [1999] and again asked by Majumdar, Neogi, Raman and Tale. The NP-completeness result improves the partial NP-completeness shown in the preliminary version of this paper published in CALDAM 2018.

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arXiv2018

Bipartitioning Problems on Graphs with Bounded Tree-Width

For an undirected graph G, we consider the following problems: given a fixed graph H, can we partition the vertices of G into two non-empty sets A and B such that neither the induced graph G[A] nor G[B] contain H (i) as a subgraph? (ii) as an induced subgraph? These problems are NP-complete and are expressible in monadic second order logic (MSOL). The MSOL formulation, together with Courcelle's theorem implies linear time solvability on graphs with bounded tree-width. This approach yields algorithms with running time f(|phi|, t) * n, where |phi| is the length of the MSOL formula, t is the tree-width of the graph and n is the number of vertices of the graph. The dependency of f(|phi|, t) on |phi| can be as bad as a tower of exponentials. In this paper, we present explicit combinatorial algorithms for these problems for graphs G whose tree-width is bounded. We obtain 2^{O(t^r)} * n time algorithms when H is any fixed graph of order r. In the special case when H = K_r, a complete graph on r vertices, we get an 2^{O(t+r \log t)} * n time algorithm. The techniques can be extended to provide FPT algorithms to determine the smallest number q such that V can be partitioned into q parts such that none of the parts have H as a subgraph (induced subgraph).

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arXiv2017

Algorithms and hardness results for happy coloring problems

In a vertex-colored graph, an edge is happy if its endpoints have the same color. Similarly, a vertex is happy if all its incident edges are happy. Motivated by the computation of homophily in social networks, we consider the algorithmic aspects of the following Maximum Happy Edges (k-MHE) problem: given a partially k-colored graph G, find an extended full k-coloring of G maximizing the number of happy edges. When we want to maximize the number of happy vertices, the problem is known as Maximum Happy Vertices (k-MHV). We further study the complexity of the problems and their weighted variants. For instance, we prove that for every k >= 3, both problems are NP-complete for bipartite graphs and k-MHV remains hard for split graphs. In terms of exact algorithms, we show both problems can be solved in time O*(2^n), and give an even faster O*(1.89^n)-time algorithm when k = 3. From a parameterized perspective, we give a linear vertex kernel for Weighted k-MHE, where edges are weighted and the goal is to obtain happy edges of at least a specified total weight. Finally, we prove both problems are solvable in polynomial-time when the graph has bounded treewidth or bounded neighborhood diversity.

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arXiv2015

Multi-threaded Graph Coloring Algorithm for Shared Memory Architecture

In this paper, we present multi-threaded algorithms for graph coloring suitable to the shared memory programming model. We modify an existing algorithm widely used in the literature and prove the correctness of the modified algorithm. We also propose a new approach to solve the problem of coloring using locks. Using datasets from real world graphs, we evaluate the performance of the algorithms on the Intel platform. We compare the performance of the sequential approach v/s our proposed approach and analyze the speedup obtained against the existing algorithm from the literature. The results show that the speedup obtained is consequential. We also provide a direction for future work towards improving the performance further in terms of different metrics.

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arXiv2014

The chromatic discrepancy of graphs

For a proper vertex coloring $c$ of a graph $G$, let $φ_c(G)$ denote the maximum, over all induced subgraphs $H$ of $G$, the difference between the chromatic number $χ(H)$ and the number of colors used by $c$ to color $H$. We define the chromatic discrepancy of a graph $G$, denoted by $φ(G)$, to be the minimum $φ_c(G)$, over all proper colorings $c$ of $G$. If $H$ is restricted to only connected induced subgraphs, we denote the corresponding parameter by $\hatφ(G)$. These parameters are aimed at studying graph colorings that use as few colors as possible in a graph and all its induced subgraphs. We study the parameters $φ(G)$ and $\hatφ(G)$ and obtain bounds on them. We obtain general bounds, as well as bounds for certain special classes of graphs including random graphs. We provide structural characterizations of graphs with $φ(G) = 0$ and graphs with $\hatφ(G) = 0$. We also show that computing these parameters is NP-hard.

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