Arc-transitive distance-regular covers of cliques with <Emphasis Type="Italic">λ</Emphasis> = µ

We study antipodal distance-regular graphs of diameter 3 such that their automorphism group acts transitively on the set of pairs (a, b), where {a, b} is an edge of the graph. Since the automorphism group of such graphs acts 2-transitively on the set of antipodal classes, the classification of 2-transitive permutation groups can be used. We classify arc-transitive distance-regular graphs of diameter 3 in which any two vertices at distance at most two have exactly µ common neighbors.     

Automorphism subgroups for designs with $$lambda =1$$ λ = 1

Designs, Codes and Cryptography - Given an integer $$kge 3$$ and a group G of odd order,&nbsp; if there exists a 2-(v,&nbsp;k,&nbsp;1)-design and if v is sufficiently large then there...     

Lifting Artin–Schreier covers with maximal wild monodromy

Let k be an algebraically closed field of characteristic p > 0. We consider the problem of lifting p-cyclic covers of ${\mathbb{P}^{1}_k}$ as p-cyclic covers C of the projective line over some discrete valuation field K under the condition that the wild monodromy is maximal. We answer positively the problem for covers birationally given by w ^p ?w = t R(t) for any additive polynomial R(t). One gives further informations about the ramification filtration of the monodromy extension and in the case when p = 2, one computes the conductor exponent f (Jac(C)/K) and the Swan conductor sw(Jac(C)/K).     

On the practical μ-stability of solutions of standard systems with delay

We study the problem of μ-stability of a dynamical system with delay. Conditions of the practical μ-stability are established for the general case and for a quasilinear system. The conditions suggested are illustrated by an example. Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 51, No. 2, pp. 204–213, February, 1999.     

Turán numbers of vertex-disjoint cliques in r-partite graphs

For two graphs $G$ and $H$, the Turán number$ex(G,H)$ is the maximum number of edges in a subgraph of $G$ that contains no copy of $H$. Chen, Li, and Tu determined the Turán numbers $ex(K_{m,n},k{K}_2)$ for all $k\ge 1$ Chen et al. (2009). In this paper we will determine the Turán numbers $ex(K_{a_1,\dots ,a_r},k{K}_r)$ for all $r\ge 3$ and $k\ge 1$.     

k L-list λ colouring of graphs

The k L-list λ colouring of a graph G is an L-list colouring (with positive integers) where any two colours assigned to adjacent vertices do not belong to a set λ, where the avoided assignments are listed. Moreover, the length of the list L(x), for every vertex x of G, must be less than or equal to a positive integer k, where k is the number of colours. This problem is NP-complete and we present an efficient heuristic algorithm to solve it. A fundamental aspect of the algorithm we developed is a particular technique of backtracking that permits the direct reassignment of the vertices causing the conflict if, at the moment of assigning a colour to a vertex, no colour on the list associated to it is available. An application of this algorithm to the problem of assigning arriving or leaving trains to the available tracks at a railway station is also discussed.     

Solvable covers of the boolean variety of unital ℓ-groups

The varieties of solvable lattice-ordered groups covering the abelian variety were shown independently by Gurchenkov, Reilly, and Darnel to be the Scrimger varieties of ?-groups and the three Medvedev representable covers. In this article, the authors give a parallel characterization of varieties of solvable unital ?-groups which cover the minimal nontrivial variety of boolean unital ?-groups.     

More covers of the Boolean variety of unital ℓ-groups

Within the lattice of varieties of pseudo MV-algebras, the variety ${\mathcal{B}}$ of Boolean algebras is the least nontrivial variety. Komori identified all varieties of (commutative) MV-algebras that cover ${\mathcal{B}}$ . The authors previously identified all solvable varieties of pseudo MV-algebras that cover ${\mathcal{B}}$ . We will show the existence of continuum many nonsolvable varieties of pseudo MV-algebras that cover ${\mathcal{B}}$ , show that periodically primitive u?-groups cannot generate Boolean covers, and show that all noncommutative varieties that are Boolean covers must be Top Boolean.     

EpGs with minimal μ: II

There are two known lower bounds for (P, Q) in an EpG, called ₁ and ₂, see for example [3]. In [4], =₁ was studied for the case of triangular EGQs and, in [3], =₂ was considered for EpGs in general. Here we extend this to the case =₁ for EpGs in general, including non-triangular EGQs, and we give a number of characterizations. For instance a triangular EpG with =₁ locally is an EGQ, an extended dual net or a semibiplane; if t>2–1, then an EpG(s, t) with =₁ locally is an EGQ. In general we have only partial results for t2–1.     

Extended partial geometries with minimal μ

Recently Fisher and Hobart extended the original work of Buekenhout and Hubaut on extended generalized quadrangles with a certain minimal value (minimal ) for the nonempty residue intersections. We show that there is another minimal which is sometimes better than the earlier one, and we extend all this to the case of extended partial geometries. We examine especially the triangular case, and find only a few new possibilities, which we are unable to settle, although some of these look extremely interesting.     

How many graphs are unions of k‐cliques?

We study the number of n‐vertex graphs that can be written as the edge‐union of k‐vertex cliques. We obtain reasonably tight estimates for in the cases (i) k = n ? o(n) and (ii) k = o(n) but . We also show that exhibits a phase transition around . We leave open several potentially interesting cases, and raise some other questions of a similar nature. © 2006 Wiley Periodicals, Inc. J Graph Theory 52: 87–107, 2006     

All Lambda-Designs With λ = 2 p are Type-1

A-design is a family B ₁,B ₂,...,B _v of subsets of X={1, 2,..., v} such that B _i B _j = for all i jand not all B _i are of the same size. Ryser's andWoodall's -design conjecture states thateach -design can be obtained from a symmetricblock design by a certain complementation procedure. Our mainresult is that the conjecture is true when is twice a prime number.     

On τ–covers using ideals and some of its consequences

In this paper we primarily introduce an ideal version of τ -covers studied in [18, 19]. We establish the inter-relationships between ?-τ -covers and ?-γ, ?-large [3, 5] and κ-covers[2, 7]. We also make some investigations involving the splittability and preservation properties. Our results extend the earlier results proved in [18, 19, 2, 7] and present a more general version with respect to ideals.