Abstract
We solve a long-standing open problem concerning a discrete mathematical model, which has various applications in computer science and several other fields, including frequency assignment and many other problems on resource allocation. A mixed hypergraph \(\mathcal H \) is a triple \((X,\mathcal C ,\mathcal D )\), where \(X\) is the set of vertices, and \(\mathcal C \) and \(\mathcal D \) are two set systems over \(X\), the families of so-called C-edges and D-edges, respectively. A vertex coloring of a mixed hypergraph \(\mathcal H \) is proper if every C-edge has two vertices with a common color and every D-edge has two vertices with different colors. A mixed hypergraph is colorable if it has at least one proper coloring; otherwise it is uncolorable. The chromatic inversion of a mixed hypergraph \(\mathcal H =(X,\mathcal C ,\mathcal D )\) is defined as \(\mathcal H ^c=(X,\mathcal D ,\mathcal C )\). Since 1995, it was an open problem wether there is a correlation between the colorability properties of a hypergraph and its chromatic inversion. In this paper we answer this question in the negative, proving that there exists no polynomial-time algorithm (provided that \(P \ne NP\)) to decide whether both \(\mathcal H \) and \(\mathcal H ^c\) are colorable, or both are uncolorable. This theorem holds already for the restricted class of 3-uniform mixed hypergraphs (i.e., where every edge has exactly three vertices). The proof is based on a new polynomial-time algorithm for coloring a special subclass of 3-uniform mixed hypergraphs. Implementation in C++ programming language has been tested. Further related decision problems are investigated, too.
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Notes
Some edges \(C\in \mathcal C ^0_\varphi \) may get their first (and currently unique) colored vertex during the extension. Formally this would mean \(C\in \mathcal C ^1_{\varphi _1}\) where \(\varphi _1\) is the one-step extension of \(\varphi \). Since \(\varphi \) will be extended in several consecutive steps, to simplify notation we have omitted the subscript of \(\mathcal C ^1\) here and also in some later text. In any case it is meant that the algorithm updates \(\mathcal C ^1\) after every single extension.
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Acknowledgments
This research was supported in part by the Hungarian Scientific Research Fund, OTKA grants 49613 and 81493.
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Hegyháti, M., Tuza, Z. Colorability of mixed hypergraphs and their chromatic inversions. J Comb Optim 25, 737–751 (2013). https://doi.org/10.1007/s10878-012-9559-7
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DOI: https://doi.org/10.1007/s10878-012-9559-7