Quasi-random Graph

	Quasi-random Graph In graph theory, a graph is said to be a pseudorandom graph if it obeys certain properties that random graphs obey with high probability. There is no concrete definition of graph pseudorandomness, but there are many reasonable characterizations of pseudorandomness one can consider. Pseudorandom properties were first formally considered by Andrew Thomason in 1987. He defined a condition called "jumbledness": a graph G=(V,E) is said to be (p,\alpha)-''jumbled'' for real p and \alpha with 0 if : $\left, e(U)-p\binom\\leq \alpha, U,$ for every subset $U$ of the vertex set $V$ , where $e(U)$ is the number of edges among $U$ (equivalently, the number of edges in the subgraph induced by the vertex set $U$ ). It can be shown that the [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
picture info	Graph Theory In mathematics, graph theory is the study of ''graphs'', which are mathematical structures used to model pairwise relations between objects. A graph in this context is made up of '' vertices'' (also called ''nodes'' or ''points'') which are connected by '' edges'' (also called ''links'' or ''lines''). A distinction is made between undirected graphs, where edges link two vertices symmetrically, and directed graphs, where edges link two vertices asymmetrically. Graphs are one of the principal objects of study in discrete mathematics. Definitions Definitions in graph theory vary. The following are some of the more basic ways of defining graphs and related mathematical structures. Graph In one restricted but very common sense of the term, a graph is an ordered pair G=(V,E) comprising: * V, a set of vertices (also called nodes or points); * E \subseteq \, a set of edges (also called links or lines), which are unordered pairs of vertices (that is, an edge is associated with t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
	Vera T Vera may refer to: Names Vera (surname), a surname (including a list of people with the name) Vera (given name), a given name (including a list of people and fictional characters with the name) *Vera (), archbishop of the archdiocese of Tarragona Places Spain Vera, Almería, a municipality in the province of Almería, Andalusia Vera de Bidasoa, a municipality in the autonomous community of Navarra La Vera, a comarca in the province of Cáceres, Extremadura United States Vera, Illinois, an unincorporated community Vera, Kansas, a ghost town * Vera, Missouri, an unincorporated community * Vera, Oklahoma, a town Vera, Texas, an unincorporated community Vera, Virginia, an unincorporated community Veradale, Washington, originally known as Vera, CDP Elsewhere Vera, Santa Fe, a city in the province of Santa Fe, Argentina Vera Department, an administrative subdivision (departamento) of the province of Santa Fe Vera, Mato Grosso, Brazil, a municipality Cape Vera, Nun ... [...More Info...] [...Related Items...] OR:* [Wikipedia] [Google] [Baidu]
	Szemerédi's Theorem In arithmetic combinatorics, Szemerédi's theorem is a result concerning arithmetic progressions in subsets of the integers. In 1936, Erdős and Turán conjectured that every set of integers ''A'' with positive natural density contains a ''k''-term arithmetic progression for every ''k''. Endre Szemerédi proved the conjecture in 1975. Statement A subset ''A'' of the natural numbers is said to have positive upper density if :\limsup_\frac > 0. Szemerédi's theorem asserts that a subset of the natural numbers with positive upper density contains infinitely many arithmetic progressions of length ''k'' for all positive integers ''k''. An often-used equivalent finitary version of the theorem states that for every positive integer ''k'' and real number \delta \in (0, 1], there exists a positive integer :N = N(k,\delta) such that every subset of of size at least δ''N'' contains an arithmetic progression of length ''k''. Another formulation uses the function ''r''''k''(''N''), the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
	Green–Tao Theorem In number theory, the Green–Tao theorem, proved by Ben Green and Terence Tao in 2004, states that the sequence of prime numbers contains arbitrarily long arithmetic progressions. In other words, for every natural number ''k'', there exist arithmetic progressions of primes with ''k'' terms. The proof is an extension of Szemerédi's theorem. The problem can be traced back to investigations of Lagrange and Waring from around 1770.. Statement Let \pi(N) denote the number of primes less than or equal to N. If A is a subset of the prime numbers such that : \limsup_ \frac>0, then for all positive integers k, the set A contains infinitely many arithmetic progressions of length k. In particular, the entire set of prime numbers contains arbitrarily long arithmetic progressions. In their later work on the generalized Hardy–Littlewood conjecture, Green and Tao stated and conditionally proved the asymptotic formula : (\mathfrak_k + o(1))\frac for the number of ''k'' tuples of primes ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
picture info	Chromatic Number In graph theory, graph coloring is a special case of graph labeling; it is an assignment of labels traditionally called "colors" to elements of a graph subject to certain constraints. In its simplest form, it is a way of coloring the vertices of a graph such that no two adjacent vertices are of the same color; this is called a vertex coloring. Similarly, an edge coloring assigns a color to each edge so that no two adjacent edges are of the same color, and a face coloring of a planar graph assigns a color to each face or region so that no two faces that share a boundary have the same color. Vertex coloring is often used to introduce graph coloring problems, since other coloring problems can be transformed into a vertex coloring instance. For example, an edge coloring of a graph is just a vertex coloring of its line graph, and a face coloring of a plane graph is just a vertex coloring of its dual. However, non-vertex coloring problems are often stated and studied as-is. This is ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
picture info	Independent Set (graph Theory) In graph theory, an independent set, stable set, coclique or anticlique is a set of vertices in a graph, no two of which are adjacent. That is, it is a set S of vertices such that for every two vertices in S, there is no edge connecting the two. Equivalently, each edge in the graph has at most one endpoint in S. A set is independent if and only if it is a clique in the graph's complement. The size of an independent set is the number of vertices it contains. Independent sets have also been called "internally stable sets", of which "stable set" is a shortening. A maximal independent set is an independent set that is not a proper subset of any other independent set. A maximum independent set is an independent set of largest possible size for a given graph G. This size is called the independence number of ''G'' and is usually denoted by \alpha(G). The optimization problem of finding such a set is called the maximum independent set problem. It is a strongly NP-hard problem. As such ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
picture info	Maximum Cut For a graph, a maximum cut is a cut whose size is at least the size of any other cut. That is, it is a partition of the graph's vertices into two complementary sets and , such that the number of edges between and is as large as possible. Finding such a cut is known as the max-cut problem. The problem can be stated simply as follows. One wants a subset of the vertex set such that the number of edges between and the complementary subset is as large as possible. Equivalently, one wants a bipartite subgraph of the graph with as many edges as possible. There is a more general version of the problem called weighted max-cut, where each edge is associated with a real number, its weight, and the objective is to maximize the total weight of the edges between and its complement rather than the number of the edges. The weighted max-cut problem allowing both positive and negative weights can be trivially transformed into a weighted minimum cut problem by flipping the sign in all weig ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
picture info	K-edge-connected Graph In graph theory, a connected graph is -edge-connected if it remains connected whenever fewer than edges are removed. The edge-connectivity of a graph is the largest for which the graph is -edge-connected. Edge connectivity and the enumeration of -edge-connected graphs was studied by Camille Jordan in 1869. Formal definition Let G = (V, E) be an arbitrary graph. If subgraph G' = (V, E \setminus X) is connected for all X \subseteq E where , X, < k, then ''G'' is ''k''-edge-connected. The edge connectivity of $G$ is the maximum value ''k'' such that ''G'' is ''k''-edge-connected. The smallest set ''X'' whose removal disconnects ''G'' is a minimum cut in ''G''. The edge connectivity version of Menger's theorem provi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
picture info	K-vertex-connected Graph In graph theory, a connected graph is said to be -vertex-connected (or -connected) if it has more than vertices and remains connected whenever fewer than vertices are removed. The vertex-connectivity, or just connectivity, of a graph is the largest for which the graph is -vertex-connected. Definitions A graph (other than a complete graph) has connectivity ''k'' if ''k'' is the size of the smallest subset of vertices such that the graph becomes disconnected if you delete them. Complete graphs are not included in this version of the definition since they cannot be disconnected by deleting vertices. The complete graph with ''n'' vertices has connectivity ''n'' − 1, as implied by the first definition. An equivalent definition is that a graph with at least two vertices is ''k''-connected if, for every pair of its vertices, it is possible to find ''k'' vertex-independent paths connecting these vertices; see Menger's theorem . This definition produces the same ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
	Ramanujan Graphs In the mathematical field of spectral graph theory, a Ramanujan graph is a regular graph whose spectral gap is almost as large as possible (see extremal graph theory). Such graphs are excellent spectral expanders. AMurty's survey papernotes, Ramanujan graphs "fuse diverse branches of pure mathematics, namely, number theory, representation theory, and algebraic geometry". These graphs are indirectly named after Srinivasa Ramanujan; their name comes from the Ramanujan–Petersson conjecture, which was used in a construction of some of these graphs. Definition Let G be a connected d-regular graph with n vertices, and let \lambda_1 \geq \lambda_2 \geq \cdots \geq \lambda_n be the eigenvalues of the adjacency matrix of G (or the spectrum of G). Because G is connected and d-regular, its eigenvalues satisfy d = \lambda_1 > \lambda_2 \geq \cdots \geq \lambda_n \geq -d . Define \lambda(G) = \max_, \lambda_i, = \max(, \lambda_2, , , \lambda_n, ). A connected d-regular graph G is a ''Ra ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
	Ramanujan Graph In the mathematical field of spectral graph theory, a Ramanujan graph is a regular graph whose spectral gap is almost as large as possible (see extremal graph theory). Such graphs are excellent spectral expanders. AMurty's survey papernotes, Ramanujan graphs "fuse diverse branches of pure mathematics, namely, number theory, representation theory, and algebraic geometry". These graphs are indirectly named after Srinivasa Ramanujan; their name comes from the Ramanujan–Petersson conjecture, which was used in a construction of some of these graphs. Definition Let G be a connected d-regular graph with n vertices, and let \lambda_1 \geq \lambda_2 \geq \cdots \geq \lambda_n be the eigenvalues of the adjacency matrix of G (or the spectrum of G). Because G is connected and d-regular, its eigenvalues satisfy d = \lambda_1 > \lambda_2 \geq \cdots \geq \lambda_n \geq -d . Define \lambda(G) = \max_, \lambda_i, = \max(, \lambda_2, , , \lambda_n, ). A connected d-regular graph G is a ''Ram ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
	Grothendieck Inequality In mathematics, the Grothendieck inequality states that there is a universal constant K_G with the following property. If ''M''''ij'' is an ''n'' × ''n'' (real or complex) matrix with : \Big, \sum_ M_ s_i t_j \Big, \le 1 for all (real or complex) numbers ''s''''i'', ''t''''j'' of absolute value at most 1, then : \Big, \sum_ M_ \langle S_i, T_j \rangle \Big, \le K_G for all vectors ''S''''i'', ''T''''j'' in the unit ball ''B''(''H'') of a (real or complex) Hilbert space ''H'', the constant K_G being independent of ''n''. For a fixed Hilbert space of dimension ''d'', the smallest constant that satisfies this property for all ''n'' × ''n'' matrices is called a Grothendieck constant and denoted K_G(d). In fact, there are two Grothendieck constants, K_G^(d) and K_G^(d), depending on whether one works with real or complex numbers, respectively. The Grothendieck inequality and Grothendieck constants are named after Alexander Grothendieck, who proved the existen ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]