By a covering of a group G we mean an epimorphism from a groupˆG to G. Introducing the notion of strong covering as a covering π : ˆ G → G such that every automorphism of G is a projection via π of an automorphism ofˆG, the main aim of this paper is to characterise double coverings which are strong. This is done in details for metacyclic groups, rotary… (More)
In this contribution the potentialities and the limitation of the silicon oxynitride (SiON) technology are reviewed and a number of integrated optical circuits and devices are presented. Mach-Zehnder, ring resonators, variable bandwidth filters, ring resonators based cross-bar matrices and a DQPSK receiver with interleaver and variable attenuators have been… (More)
Isometric foldings are a special class of length-preserving maps of Riemannian manifolds and were initially studied by S. Robertson. For an explanation of their topological and combinatorial properties, see the related works of we explore some properties of the singular set and describe the image set of planar, spherical, and hyperbolic foldings.