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Complement Submodular Information Measures for Balanced and Robust Data Selection On The Morse Ensemble Polynomial Of Simplicial Complexes 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
Split-Twin Extensions Preserving Seymour Vertices
Stanisław M. S. Halkiewicz · 2026-01-29 · via math.CO updates on arXiv.org

The Second Neighborhood Conjecture of Seymour asserts that every oriented graph contains a vertex~$v$ satisfying $|\Npp(v)|\ge|\Np(v)|$. We introduce \emph{Pisa graphs} -- strongly connected oriented graphs~$D$ with $Δ(D)=\max_{v\in V(D)}\bigl(|\Npp(v)|-|\Np(v)|\bigr)=0$ -- named after the Leaning Tower of Pisa, as these graphs stand at the precise boundary between satisfying and potentially violating the conjecture. We prove that a Pisa graph containing a vertex of outdegree one must have underlying graph~$C_n$. We verify computationally that every Pisa graph on at most seven vertices has underlying graph isomorphic to either~$C_n$ or~$K_n$ minus a matching, and conjecture this holds in general. Partial structural results are presented, including a decomposition formula for the sum of all vertex margins, and a connection to blowup constructions for potential counterexamples due to Zelenskyi, Darmosiuk and Nalivayko.