Searcharxiv⌕ Search

arXiv subjects

Vladimir Nikiforov

Publications and source records attributed to Vladimir Nikiforov.

At least 37 records · Page 2Linked to original sources
arXiv2013

Maxima of the Q-index: graphs with bounded clique number

This paper gives a tight upper bound on the spectral radius of the signless Laplacian of graphs of given order and clique number. More precisely, let G be a graph of order n, let A be its adjacency matrix, and let D be the diagonal matrix of the row-sums of A. If G has clique number r, then the largest eigenvalue q(G) of the matrix Q=A+D satisfies q(G)<= 2(1-1/r)n. If G is a complete regular r-partite graph, then equality holds in the above inequality. This result confirms a conjecture of Hansen and Lucas.

math.CO↗
arXiv2013

Maxima of the Q-index: graphs without long paths

This paper gives tight upper bound on the largest eigenvalue q(G) of the signless Laplacian of graphs with no paths of given order. The main ingredient of our proof is a stability result of its own interest, about graphs with large minimum degree and with no long paths. This result extends previous work of Ali and Staton.

math.CO↗
arXiv2013

Analytic methods for uniform hypergraphs

This paper develops analityc methods for investigating uniform hypergraphs. Its starting point is the spectral theory of 2-graphs, in particular, the largest and the smallest eigenvalues of 2-graphs. On the one hand, this simple setup is extended to weighted r-graphs, and on the other, the eigenvalues-numbers are generalized to eigenvalues-functions, which encompass also other graph parameters like Lagrangians and number of edges. The resulting theory is new even for 2-graphs, where well-settled topics become challenges again. The paper covers a multitude of topics, with more than a hundred concrete statements to underpin an analytic theory for hypergraphs. Essential among these topics are a Perron-Frobenius type theory and methods for extremal hypergraph problems. Many open problems are raised and directions for possible further research are outlined.

math.CO↗
arXiv2013

Maxima of the Q-index: forbidden 4-cycle and 5-cycle

This paper gives tight upper bounds on the largest eigenvalue q(G) of the signless Laplacian of graphs with no 4-cycle and no 5-cycle. If n is odd, let F_{n} be the friendship graph of order n; if n is even, let F_{n} be F_{n-1} with an edge hanged to its center. It is shown that if G is a graph of order n, with no 4-cycle, then q(G)<q(F_{n}), unless G=F_{n}. Let S_{n,k} be the join of a complete graph of order k and an independent set of order n-k. It is shown that if G is a graph of order n, with no 5-cycle, then q(G)<q(S_{n,2}), unless G=S_{n,k}. It is shown that these results are significant in spectral extremal graph problems. Two conjectures are formulated for the maximum q(G) of graphs with forbidden cycles.

math.CO↗
arXiv2013

An analytic theory of extremal hypergraph problems

In this paper extremal problems for uniform hypergraphs are studied in the general setting of hereditary properties. It turns out that extremal problems about edges are particular cases of a general analyic problem about a recently introduced graph parameter. The paper builds a basis for the systematic study of this parameter and illustrates a range of various proof tools. It is shown that extremal problems about the number of edges of uniform hypergraphs are asymptotically equivalent to extremal problems about the largest eigenvalue; this result is new even for 2-graphs. Several concrete problems are adressed and solutions to many more are suggested. A number of open problems are raised and directions for further studies are outlined.

math.CO↗
arXiv2013

Some extremal problems for hereditary properties of graphs

This note answers extremal questions like: what is the maximum number of edges in a graph of order n, which belongs to some hereditary property. The same question is answered also for the spectral radius and other similar parameters.

math.CO↗
arXiv2013

An extension of Maclaurin's inequalities

We give a combinatorial extension of the classical inequalities of Maclaurin about symmetric functions of several variables. We discuss two problems - one analytical and another combinatorial - and show that they are in some sense equaivalent. These results complete and extend earlier results of Motzkin and Straus, Khadzhiivanov, Sos and Straus, Fisher and Ryan, and Petingi and Rodriguez.

math.CO↗
arXiv2013

Maximum norms of graphs and matrices, and their complements

In this paper, we mainly study the trace norm of the adjacency matrix of a graph, also known as the energy of graph. We give the maximum trace norms for the graph and its complement. In fact, the above problem is stated and solved in a more general setup - for nonnegative matrices with bounded entries. In particular, this study exhibits analytical matrix functions attaining maxima on matrices with rigid and complex combinatorial structure. In the last section the same questions are studied for Ky Fan norms. Possibe directions for further research are outlined, as it turns out that the above problems are just a tip of a larger multidimensional research area.

math.FA↗
arXiv2012

On the second largest eigenvalue of the signless Laplacian

Let $G$ be a graph of order $n,$ and let $q_{1}(G) \geq ...\geq q_{n}(G) $ be the eigenvalues of the $Q$-matrix of $G$, also known as the signless Laplacian of $G.$ In this paper we give a necessary and sufficient condition for the equality $q_{k}(G) =n-2,$ where $1<k\leq n.$ In particular, this result solves an open problem raised by Wang, Belardo, Huang and Borovicanin. We also show that [ q_{2}(G) \geqδ(G)] and determine that equality holds if and only if $G$ is one of the following graphs: a star, a complete regular multipartite graph, the graph $K_{1,3,3},$ or a complete multipartite graph of the type $K_{1,...,1,2,...,2}$.

math.SP↗
arXiv2011

Some new results in extremal graph theory

In recent years several classical results in extremal graph theory have been improved in a uniform way and their proofs have been simplified and streamlined. These results include a new Erdős-Stone-Bollobás theorem, several stability theorems, several saturation results and bounds for the number of graphs with large forbidden subgraphs. Another recent trend is the expansion of spectral extremal graph theory, in which extremal properties of graphs are studied by means of eigenvalues of various matrices. One particular achievement in this area is the casting of the central results above in spectral terms, often with additional enhancement. In addition, new, specific spectral results were found that have no conventional analogs. All of the above material is scattered throughout various journals, and since it may be of some interest, the purpose of this survey is to present the best of these results in a uniform, structured setting, together with some discussions of the underpinning ideas.

math.CO↗
arXiv2010

Extremal norms of graphs and matrices

In the recent years, the trace norm of graphs has been extensively studied under the name of graph energy. In this paper some of this research is extended to more general matrix norms, like the Schatten p-norms and the Ky Fan k-norms. Whenever possible the results are given both for graphs and general matrices. In various contexts a puzzling fact was observed: the Schatten p-norms are widely different for 1<=p<2 and for p>=2.

math.CO↗
arXiv2010

On the sum of k largest singular values of graphs and matrices

In the recent years, the trace norm of graphs has been extensively studied under the name of graph energy. The trace norm is just one of the Ky Fan k-norms, given by the sum of the k largest singular values, which are studied more generally in the present paper. Several relations to chromatic number, spectral radius, spread, and to other fundamental parameters are outlined. Some results are extended to more general matrices.

math.CO↗
arXiv2010

The number of graphs with large forbidden subgraphs

In this note, extending some results of Erdos, Frankl, Rodl, Alexeev, Bollobas and Thomason we determine asymptotically the number of graphs which do not contain certain large subgraphs. In particular, if H_1,...,H_n,... are graphs with chromatic numbers r_1,...,r_n,... and order o(log n), we dermine asymptotically the number of graphs of order n not containing H_n as a subgraph. We also give similar results for induced subgraphs.

math.CO↗
arXiv2010

Large joints in graphs

We show that if G is a graph of sufficiently large order n containing as many r-cliques as the r-partite Turan graph of order n; then for some C>0 G has more than Cn^(r-1) (r+1)-cliques sharing a common edge unless G is isomorphic to the the r-partite Turan graph of order n. This structural result generalizes a previous result that has been useful in extremal graph theory.

math.CO↗
arXiv2010

A new type of Ramsey-Turan problems

We introduce and study a new type of Ramsey-Turan problems, a typical example of which is the following one: let c>0 and G be a graph of sufficiently large order n with minimum degree >3n/4. If the edges of G are colored in blue or red, then for every 3<k<(1/8-c)n, there exists a monochromatic cycle of length k.

math.CO↗
arXiv2009

Cut-norms and spectra of matrices

One of the aims of this paper is to solve an open problem of Lovasz about relations between graph spectra and cut-distance. The paper starts with several inequalities between two versions of the cut-norm and the two largest singular values of arbitrary complex matrices, exteding, in particular, the well-known graph-theoretical Expander Mixing Lemma and giving a hitherto unknown converse of it. Next, cut-distance is defined for Hermitian matrices, and, separately, for arbitrary complex matrices; using these extensions, we give upper bounds on the difference of corresponding eigenvalues and singular values of two matrices, thus solving the problem of Lovasz. Finally, we deduce a spectral sampling theorem, which informally states that almost all principal submatrices of a real symmetric matrix are spectrally similar to it.

math.FA↗
arXiv2009

A contribution to the Zarankiewicz problem

Given positive integers m,n,s,t, let z(m,n,s,t) be the maximum number of ones in a (0,1) matrix of size m-by-n that does not contain an all ones submatrix of size s-by-t. We find a flexible upper bound on z(m,n,s,t) that implies the known bounds of Kovari, Sos and Turan, and of Furedi. As a consequence, we find an upper bound on the spectral radius of a graph of order n without a complete bipartite subgraph K_{s,t}.

math.CO↗