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Complement Submodular Information Measures for Balanced and Robust Data Selection A Proof of a Conjecture on Positive and Negative Square Energies of Unicyclic Graphs Laplacian Spectrum of the Weakly Zero-Divisor Graph of a Finite Commutative Ring An identity for second Eulerian numbers via lattice-point counting $t$-tone edge coloring of graphs Constructing Maximal Bumpless Pipedreams for Double Grothendieck Polynomials Mubayi's Polynomial-Ideal Conjecture and Cover-Ideal Turán Methods Implicit Binarization via Complex Phase Dynamics in Combinatorial Optimization The limits of Schur multipliers in Pólya conversion problems for the $q$-permanent function Universality theorems for generalized splines Framing Triangulations for Arbitrary Integer Flow Polytopes On the Common Generalization of Gentle Algebras and Framed Directed Acyclic Graphs The complexity of frugal digraph homomorphisms Chaotic and periodic behavior of jeu de taquin on infinite Young tableaux Enumerating Pattern Avoiding Parking Functions Incidence toric ideals and three-point functions Unique Winning Opening Move in Three-Row Chomp Strong majority colorings of graphs A Balancing Theorem for Spanning Trees of Rectangular Grid Graphs Spectral radius and edge-disjoint connected factors of graphs New invariants for rank metric codes, with applications to the classification of rank two semifields of order 256 Flexible DP-4-coloring of planar graphs without 4-cycles and intersecting triangles Balanced intersection size distributions in projective planes List Reconstruction Problem with List Size Two Is Dimensionality a Barrier for Retrieval Models? The INIEP: Irreducible and Positive Realizations The number of Pfaffian orientations on punctured polygonally cellulated surfaces Explicit Construction of Polytopes whose Ehrhart Polynomials Realize any Given Sign Pattern Finite-state enumeration of adjacency-constrained 132-avoiding permutations AMDS and quantum AMDS Constacyclic codes of length $4p^ς$ over $\mathbb{F}_{{p}^{m}}$
Between proper and square colorings of sparse graphs
Ilkyoo Choi, Xujun Liu · 2025-09-03 · via math.CO updates on arXiv.org

An $i$-independent set is a set of vertices whose pairwise distance is at least $i+1$. A proper coloring (resp. a square coloring) of a graph is a partition of its vertices into independent (resp. $2$-independent) sets. A packing $(1^{\ell},2^k)$-coloring of a graph is a partition of its vertices into $\ell$ independent sets and $k$ $2$-independent sets; this is an intermediate coloring between proper coloring and square coloring. We investigate classes of sparse graphs that have a proper $(\ell+1)$-coloring but no packing $(1^{\ell},2^k)$-coloring for any finite $k$. The Four Color Theorem states that every planar graph is packing $(1^4)$-colorable, and Grötzsch's Theorem says every planar graph with girth at least $4$ is packing $(1^3)$-colorable. However, for every fixed $k$, we construct a planar graph with no packing $(1^{3},2^k)$-coloring and a planar graph with girth $6$ that has no packing $(1^{2},2^k)$-coloring. Moreover, for every positive integer $\ell$, we completely determine the minimum girth condition $g(\ell)$ for which every planar graph with girth at least $g(\ell)$ has a packing $(1^{\ell},2^{f(\ell)})$-coloring for some finite $f(\ell)$. Our results are actually in terms of maximum average degree. We also study the list version of packing colorings. We extend two results of Gastineau and Togni by showing every subcubic graph is both packing $(1^{1},2^6)$-choosable and packing $(1^{2},2^3)$-choosable, and our results are sharp. In addition, we strengthen Voigt's example of a planar graph that is not $4$-choosable by constructing a planar graph that is not packing $(1^{4},2^k)$-choosable for every positive integer $k$.