Abstract One of the basic assumptions in hub covering problems is considering the covering radius as an exogenous parameter which cannot be controlled by the decision maker. Practically and in many real world cases with a negligible increase in costs, to increase the covering radii, it is possible to save the costs of establishing additional hub nodes. Change in problem parameters during the pl...

We consider the problem of determining the conformai type—hyperbolic or parabolic—of a covering surface of the Riemann sphere with n punctures. To such a surface there corresponds a graph called the Speiser graph of the covering, and it is natural to ask for a criterion for the type of the surface in terms of properties of the graph. We show how to define a random walk on the vertices of the gr...

Our study of the gyroid minimal surface has revealed that there are two distinct covering maps from the hyperbolic plane onto the surface that respect its intrinsic symmetries. We show that if a decoration of H is chiral, the projection of this pattern via the two covering maps gives rise to distinct structures in E. PACS. 61.50.Ah Theory of crystal structure, crystal symmetry; calculations and...

Given a branched covering between closed connected surfaces, one can easily establish some relations between the Euler characteristic and orientability of the involved surfaces, the degree of the covering, the number of branching points, and the local degrees at these points. These relations can therefore be regarded as necessary conditions for the existence of the covering. A classical problem...

Let π1 : K H , π2 : H G and π2π1 : K G be three finite regular coverings of graphs, and let σ be a representation of the covering transformation group of π1. We show that the L-function of G associated to the representation of the covering transformation group of π2π1 induced from σ is equal to that of H associated to σ by means of ordinary voltage assignments. © 2003 Elsevier Science Ltd. All ...

If M is compact Kählerian and ρ is semi-simple, that is that the Zariski closure of the image of ρ in GL(n,C) is semi-simple 1, by means of a result of Donaldson (in the case of Riemann surfaces, [5]) and Corlette (in the higher dimensional case, [4]), there exists a unique harmonic metric u on V, equivalently, a ρ-equivariant harmonic map from the universal covering M̃ of M into Pn := GL(n,C)/U...