Trees with Given Stability Number and Minimum Number of Stable Sets

We study the structure of trees minimizing their number of stable sets for given order n and stability number α. Our main result is that the edges of a non-trivial extremal tree can be partitioned into n ? α stars, each of size \({\lceil\frac{n-1}{n-\alpha}\rceil}\) or \({\lfloor\frac{n-1}{n-\alpha}\rfloor}\) , so that every vertex is included in at most two distinct stars, and the centers of these stars form a stable set of the tree.     

An upper bound for the number of uniformly packed binary codes

Under study are the binary codes uniformly packed (in the wide sense) of length n with minimum distance d and covering radius ρ. It is shown that every code of this kind is uniquely determined by the set of its codewords of weights ?n/2? ? ρ, …, ?n/2? + ρ. For odd d, the number of distinct codes is at most

$2^{2^{n - \tfrac{3}{2}\log n + o(log n)} } $

.

     

The Best Mixing Time for Random Walks on Trees

We characterize the extremal structures for mixing walks on trees that start from the most advantageous vertex. Let \(G=(V,E)\) be a tree with stationary distribution \(\pi \). For a vertex \(v \in V\), let \(H(v,\pi )\) denote the expected length of an optimal stopping rule from v to \(\pi \). The best mixing time for G is \(\min _{v \in V} H(v,\pi )\). We show that among all trees with \(|V|=n\), the best mixing time is minimized uniquely by the star. For even n, the best mixing time is maximized by the uniquely path. Surprising, for odd n, the best mixing time is maximized uniquely by a path of length \(n-1\) with a single leaf adjacent to one central vertex.     

On short cycles in triangle-free oriented graphs

An orientation of a simple graph is referred to as an oriented graph. Caccetta and Häggkvist conjectured that any digraph on n vertices with minimum outdegree d contains a directed cycle of length at most ?n/d?. In this paper, we consider short cycles in oriented graphs without directed triangles. Suppose that α0 is the smallest real such that every n-vertex digraph with minimum outdegree at least α₀n contains a directed triangle. Let ε < (3 ? 2α₀)/(4 ? 2α₀) be a positive real. We show that if D is an oriented graph without directed triangles and has minimum outdegree and minimum indegree at least (1/(4 ? 2α₀)+ε)|D|, then each vertex of D is contained in a directed cycle of length l for each 4 ≤ l < (4 ? 2α₀)ε|D|/(3 ? 2α₀) + 2.     

Diversity vectors of balls in graphs and estimates of the components of the vectors

The diversity vectors of balls are considered (the ith component of a vector of this kind is equal to the number of different balls of radius i) for the usual connected graphs and the properties of the components of the vectors are studied. The sharp upper and lower estimates are obtained for the number of different balls of a given radius in the n-vertex graphs (trees) and n-vertex trees (graphs with n ? 2d) of diameter d. It is shown that the estimates are precise in every graph regardless of the radius of balls. It is proven a necessary and sufficient condition is given for the existence of an n-vertex graph of diameter d with local (complete) diversity of balls.     

Distance domination of generalized de Bruijn and Kautz digraphs

Let G = (V,A) be a digraph and k ≥ 1 an integer. For u, v ∈ V, we say that the vertex u distance k-dominate v if the distance from u to v at most k. A set D of vertices in G is a distance k-dominating set if each vertex of V D is distance k-dominated by some vertex of D. The distance k-domination number of G, denoted by γ _k (G), is the minimum cardinality of a distance k-dominating set of G. Generalized de Bruijn digraphs G _B (n, d) and generalized Kautz digraphs G _K (n, d) are good candidates for interconnection networks. Denote Δ _k := (∑ _j=0 ^k d ^j )^?1. F. Tian and J. Xu showed that ?nΔ _k ? γ _k (G _B (n, d)) ≤?n/d ^k? and ?nΔ _k ? ≤ γ _k (G _K (n, d)) ≤ ?n/d ^k ?. In this paper, we prove that every generalized de Bruijn digraph G _B (n, d) has the distance k-domination number ?nΔ _k ? or ?nΔ _k ?+1, and the distance k-domination number of every generalized Kautz digraph G _K (n, d) bounded above by ?n/(d ^k?1+d ^k )?. Additionally, we present various sufficient conditions for γ _k (G _B (n, d)) = ?nΔ _k ? and γ _k (G _K (n, d)) = ?nΔ _k ?.     

Super Edge-connectivity and Zeroth-order General Randić Index for −1 ≤ <Emphasis Type="Italic">α</Emphasis> &lt; 0

Let G be a connected graph with order n, minimum degree δ = δ(G) and edge-connectivity λ = λ(G). A graph G is maximally edge-connected if λ = δ, and super edge-connected if every minimum edgecut consists of edges incident with a vertex of minimum degree. Define the zeroth-order general Randi? index \(R_\alpha ^0\left( G \right) = \sum\limits_{x \in V\left( G \right)} {d_G^\alpha \left( x \right)} \), where d_G(x) denotes the degree of the vertex x. In this paper, we present two sufficient conditions for graphs and triangle-free graphs to be super edge-connected in terms of the zeroth-order general Randi? index for ?1 ≤ α < 0, respectively.     

On Tverberg partitions

A theorem of Tverberg from 1966 asserts that every set X ? ? ^d of n = T(d, r) = (d + 1)(r ? 1) + 1 points can be partitioned into r pairwise disjoint subsets, whose convex hulls have a point in common. Thus every such partition induces an integer partition of n into r parts (that is, r integers a ₁,..., a _r satisfying n = a ₁ + ··· + a _r ), in which the parts a _i correspond to the number of points in every subset. In this paper, we prove that for any partition of n where the parts satisfy a _i ≤ d + 1 for all i = 1,..., r, there exists a set X ? ? of n points, such that every Tverberg partition of X induces the same partition on n, given by the parts a ₁,..., a _r .     

(3, 1)*-Choosability of graphs of nonnegative characteristic without intersecting short cycles

A graph G is called (k,d)^?-choosable if for every list assignment L satisfying ∣L(v)∣ ≥k for all v∈V(G), there is an L-coloring of G such that each vertex of G has at most d neighbors colored with the same color as itself. In this paper, it is proved that every graph of nonnegative characteristic without intersecting i-cycles for all i=3,4,5 is (3,1)^?-choosable.     

The real symmetric matrices of odd order with a P-set of maximum size

Suppose that A is a real symmetric matrix of order n. Denote by m_A(0) the nullity of A. For a nonempty subset α of {1, 2,..., n}, let A(α) be the principal submatrix of A obtained from A by deleting the rows and columns indexed by α. When m_A(α)(0) = m_A(0)+|α|, we call α a P-set of A. It is known that every P-set of A contains at most ?n/2? elements. The graphs of even order for which one can find a matrix attaining this bound are now completely characterized. However, the odd case turned out to be more difficult to tackle. As a first step to the full characterization of these graphs of odd order, we establish some conditions for such graphs G under which there is a real symmetric matrix A whose graph is G and contains a P-set of size (n ? 1)/2.     

Edge clique covering sum of graphs

The edge clique cover sum number (resp. edge clique partition sum number) of a graph G, denoted by scc(G) (resp. scp(G)), is defined as the smallest integer k for which there exists a collection of complete subgraphs of G, covering (resp. partitioning) all edges of G such that the sum of sizes of the cliques is at most k. By definition, scc(G) \({\leqq}\) scp(G). Also, it is known that for every graph G on n vertices, scp(G) \({\leqq n^{2}/2}\). In this paper, among some other results, we improve this bound for scc(G). In particular, we prove that if G is a graph on n vertices with no isolated vertex and the maximum degree of the complement of G is d ? 1, for some integer d, then scc(G) \({\leqq cnd\left\lceil\log \left(({n-1})/(d-1)\right)\right\rceil}\), where c is a constant. Moreover, we conjecture that this bound is best possible up to a constant factor. Using a well-known result by Bollobás on set systems, we prove that this conjecture is true at least for d = 2. Finally, we give an interpretation of this conjecture as an interesting set system problem which can be viewed as a multipartite generalization of Bollobás’ two families theorem.     

Construction of lattice rules with a trigonometric <Emphasis Type="Italic">d</Emphasis>-property on the basis of extreme lattices

Lattice rules with the trigonometric d-property that are optimal with respect to the number of points are constructed for the approximation of integrals over an n-dimensional unit cube. An extreme lattice for a hyperoctahedron at n = 4 is used to construct lattice rules with the trigonometric d-property and the number of points

$0.80822 \ldots \cdot \Delta ^4 (1 + o(1)), \Delta \to \infty $

(d = 2Δ ? 1 ≥ 3 is an arbitrary odd number). With few exceptions, the resulting lattice rules have the highest previously known effectiveness factor.

     

Improved Upper Bound for Generalized Acyclic Chromatic Number of Graphs

A vertex coloring of a graph G is called r-acyclic if it is a proper vertex coloring such that every cycle D receives at least min{|D|, r} colors. The r-acyclic chromatic number of G is the least number of colors in an r-acyclic coloring of G. We prove that for any number r ≥ 4, the r-acyclic chromatic number of any graph G with maximum degree Δ ≥ 7 and with girth at least (r ? 1)Δ is at most (4r ? 3)Δ.     

Intersecting Convex Sets by Rays

What is the smallest number τ=τ(n) such that for any collection of n pairwise disjoint convex sets in d-dimensional Euclidean space, there is a point such that any ray (half-line) emanating from it meets at most τ sets of the collection? This question of Urrutia is closely related to the notion of regression depth introduced by Rousseeuw and Hubert (1996). We show the following:Given any collection \({\mathcal{C}}\) of n pairwise disjoint compact convex sets in d-dimensional Euclidean space, there exists a point p such that any ray emanating from p meets at most \(\frac{dn+1}{d+1}\) members of \({\mathcal{C}}\).There exist collections of n pairwise disjoint (i) equal-length segments or (ii) disks in the Euclidean plane such that from any point there is a ray that meets at least \(\frac{2n}{3}-2\) of them.We also determine the asymptotic behavior of τ(n) when the convex bodies are fat and of roughly equal size.     

Two-weighted estimates for generalized fractional maximal operators on non-homogeneous spaces

Let μ be a nonnegative Borel measure on R ^d satisfying that μ(Q) ? l(Q)ⁿ for every cube Q ? R ⁿ , where l(Q) is the side length of the cube Q and 0 < n ? d.We study the class of pairs of weights related to the boundedness of radial maximal operators of fractional type associated to a Young function B in the context of non-homogeneous spaces related to the measure μ. Our results include two-weighted norm and weak type inequalities and pointwise estimates. Particularly, we give an improvement of a two-weighted result for certain fractional maximal operator proved in W.Wang, C. Tan, Z. Lou (2012).     

Some estimates for the number of vertices of integer polyhedra

A polyhedron is called integer if its every vertex has integer coordinates. We consider integer polyhedra P _I = conv(P ∩ ? ^d ) defined implicitly; that is, no system of linear inequalities is known for P _I but some is known for P. Some estimates are given for the number of vertices of P _I .     

Vertex-fault-tolerant cycles embedding on enhanced hypercube networks

In this paper, we focus on the vertex-fault-tolerant cycles embedding on enhanced hypercube, which is an attractive variant of hypercube and is obtained by adding some complementary edges from hypercube. Let F _v be the set of faulty vertices in the n-dimensional enhanced hypercube Q _n,k (1 ≤ k ≤ n?1). When |F _v | = 2, we showed that Q _n,k ? F _v contains a fault-free cycle of every even length from 4 to 2 ⁿ ?4 where n (n ≥ 3) and k have the same parity; and contains a fault-free cycle of every even length from 4 to 2 ⁿ ? 4, simultaneously, contains a cycle of every odd length from n ? k + 2 to 2 ⁿ ? 3 where n(≥ 3) and k have the different parity. Furthermore, when |F ^v | = f ^v ≤ n ? 2, we proof that there exists the longest fault-free cycle, which is of even length 2 ⁿ ? 2f ^v whether n(n ≥ 3) and k have the same parity or not; and there exists the longest fault-free cycle, which is of odd length 2 ⁿ ? 2f ^v ? 1 in Q ^n,k ? F ^v where n(≥ 3) and k have the different parity.     

On the tree structure of the power digraphs modulo n

For any two positive integers n and k ? 2, let G(n, k) be a digraph whose set of vertices is {0, 1, …, n ? 1} and such that there is a directed edge from a vertex a to a vertex b if a ^k ≡ b (mod n). Let \(n = \prod\nolimits_{i = 1}^r {p_i^{{e_i}}} \) be the prime factorization of n. Let P be the set of all primes dividing n and let P ₁, P ₂ ? P be such that P ₁ ∪ P ₂ = P and P ₁ ∩ P ₂ = ?. A fundamental constituent of G(n, k), denoted by \(G_{{P_2}}^*(n,k)\), is a subdigraph of G(n, k) induced on the set of vertices which are multiples of \(\prod\nolimits_{{p_i} \in {P_2}} {{p_i}} \) and are relatively prime to all primes q ∈ P ₁. L. Somer and M. K?i?ek proved that the trees attached to all cycle vertices in the same fundamental constituent of G(n, k) are isomorphic. In this paper, we characterize all digraphs G(n, k) such that the trees attached to all cycle vertices in different fundamental constituents of G(n, k) are isomorphic. We also provide a necessary and sufficient condition on G(n, k) such that the trees attached to all cycle vertices in G(n, k) are isomorphic.     

Super (<Emphasis Type="Italic">a</Emphasis>, <Emphasis Type="Italic">d</Emphasis>)-edge-antimagic total labelings of complete bipartite graphs

An (a, d)-edge-antimagic total labeling of a graph G is a bijection f from V(G) ∪ E(G) onto {1, 2,…,|V(G)| + |E(G)|} with the property that the edge-weight set {f(x) + f(xy) + f(y) | xy ∈ E(G)} is equal to {a, a + d, a + 2d,...,a + (|E(G)| ? 1)d} for two integers a > 0 and d ? 0. An (a, d)-edge-antimagic total labeling is called super if the smallest possible labels appear on the vertices. In this paper, we completely settle the problem of the super (a, d)-edge-antimagic total labeling of the complete bipartite graph K_m,n and obtain the following results: the graph K_m,n has a super (a, d)-edge-antimagic total labeling if and only if either (i) m = 1, n = 1, and d ? 0, or (ii) m = 1, n ? 2 (or n = 1 and m ? 2), and d ∈ {0, 1, 2}, or (iii) m = 1, n = 2 (or n = 1 and m = 2), and d = 3, or (iv) m, n ? 2, and d = 1.     

An exact algorithm for finding a vector subset with the longest sum

We consider the problem: Given a set of n vectors in the d-dimensional Euclidean space, find a subsetmaximizing the length of the sum vector.We propose an algorithm that finds an optimal solution to this problem in time O(n^d?1(d + logn)). In particular, if the input vectors lie in a plane then the problem is solvable in almost linear time.