Unbounded regions of infinitely logconcave sequences.
Undecidability and Nonperiodicity for Tilings of the Plane.
Undecidable Tiling Problems in the Hyperbolic Plane.
Underlying Graphs of 3-Quasi-Transitive Digraphs and 3-Transitive Digraphs
A digraph is 3-quasi-transitive (resp. 3-transitive), if for any path x0x1 x2x3 of length 3, x0 and x3 are adjacent (resp. x0 dominates x3). C´esar Hern´andez-Cruz conjectured that if D is a 3-quasi-transitive digraph, then the underlying graph of D, UG(D), admits a 3-transitive orientation. In this paper, we shall prove that the conjecture is true.
Undirected and directed graphs with near polynomial growth
The growth function of a graph with respect to a vertex is near polynomial if there exists a polynomial bounding it above for infinitely many positive integers. In the paper vertex-symmetric undirected graphs and vertex-symmetric directed graphs with coinciding in- and out-degrees are described in the case their growth functions are near polynomial.
Undirected strict gammoids
Une application des variables bivalentes aux BIBD
Une approche métrique de la rétraction dans les ensembles ordonnés et les graphes
Une axiomatisation au premier ordre des arrangements de pseudodroites euclidiennes
Nous définissons une structure logique permettant de représenter les classes d’homéomorphismes des arrangements de pseudodroites du plan euclidien. Nous donnons une axiomatisation finie du premier ordre de la réalisabilité des arrangements de pseudodroites.
Une bijection entre les polyominos convexes dirigés et les mots de Dyck bilatères
Une construction du groupe de Fischer
Une heuristique pour le problème de l'arbre de Steiner
Une opérade anticyclique sur les arbustes
We define new combinatorial objects, called shrubs, such that forests of rooted trees are shrubs. We then introduce a structure of operad on shrubs. We show that this operad is contained in the Zinbiel operad, by using the inclusion of Zinbiel in the operad of moulds. We also prove that this inclusion is compatible with the richer structure of anticyclic operad that exists on Zinbiel and on moulds.
Une preuve combinatoire de la rationalité d'une série génératrice associée aux arbres
Une propriété de symétrie des nombres de Genocchi
Unicyclic graphs with bicyclic inverses
A graph is nonsingular if its adjacency matrix is nonsingular. The inverse of a nonsingular graph is a graph whose adjacency matrix is similar to via a particular type of similarity. Let denote the class of connected bipartite graphs with unique perfect matchings. Tifenbach and Kirkland (2009) characterized the unicyclic graphs in which possess unicyclic inverses. We present a characterization of unicyclic graphs in which possess bicyclic inverses.
Unicyclic graphs with the strong reciprocal eigenvalue property.
Unification of the quintuple and septuple product identities.
Unification of zero-sum problems, subset sums and covers of .
Unified Spectral Bounds on the Chromatic Number
One of the best known results in spectral graph theory is the following lower bound on the chromatic number due to Alan Hoffman, where μ1 and μn are respectively the maximum and minimum eigenvalues of the adjacency matrix: χ ≥ 1+μ1/−μn. We recently generalised this bound to include all eigenvalues of the adjacency matrix. In this paper, we further generalize these results to include all eigenvalues of the adjacency, Laplacian and signless Laplacian matrices. The various known bounds are also unified...