arXiv subjects

Josef Lauri

Publications and source records attributed to Josef Lauri.

24 records · Page 2Linked to original sources

arXiv2014

A Generalisation of Isomorphisms with Applications

In this paper, we study the behaviour of TF-isomorphisms, a natural generalisation of isomorphisms. TF-isomorphisms allow us to simplify the approach to seemingly unrelated problems. In particular, we mention the Neighbourhood Reconstruction problem, the Matrix Symmetrization problem and Stability of Graphs. We start with a study of invariance under TF-isomorphisms. In particular, we show that alternating trails and incidence double covers are conserved by TF-isomorphisms, irrespective of whether they are TF-isomorphisms between graphs or digraphs. We then define an equivalence relation and subsequently relate its equivalence classes to the incidence double cover of a graph. By directing the edges of an incidence double cover from one colour class to the other and discarding isolated vertices we obtain an invariant under TF-isomorphisms which gathers a number of invariants. This can be used to study TF-orbitals, an analogous generalisation of the orbitals of a permutation group.

math.CO↗

arXiv2014

$(2,2)$-colourings and clique-free $σ$-hypergraphs

We consider vertex colourings of $r$-uniform hypergraphs $H$ in the classical sense, that is such that no edge has all its vertices given the same colour, and $(2,2)$-colourings of $H$ in which the vertices in any edge are given exactly two colours. This is a special case of constrained colourings introduced by Bujtas and Tuza which, in turn, is a generalisation of Voloshin's colourings of mixed hypergraphs. We study, $χ(H)$, the classical chromatic number, and the $(2,2)$-spectrum of $H$, that is, the set of integers $k$ for which $H$ has a $(2,2)$-colouring using exactly $k$ colours. We present extensions of hypergraphs which preserve both the chromatic number and the $(2,2)$-spectrum and which, however often repeated, do not increase the clique number of $H$ by more than a fixed number. In particular, we present sparse $(2,2)$-colourable clique-free $σ$-hypergraphs having arbitrarily large chromatic number - these $r$-uniform hypergraphs were studied by the authors in earlier papers. We use these ideas to extend some known $3$-uniform hypergraphs which exhibit a $(2,2)$-spectrum with remarkable gaps. We believe that this work is the first to present an extension of hypergraphs which preserves both $χ(H)$ and the $(2,2)$-spectrum of $H$ simultaneously.

math.CO↗

arXiv2014

Unstable Graphs: A Fresh Outlook via TF-Automorphisms

In this paper, we first establish the very close link between stability of graphs, a concept first introduced in \cite{Scapsalvi1} and studied most notably by Surowski \cite{Surowski1}, \cite{Surowski2} and Wilson \cite{Wilson01} and two-fold automorphisms. The concept of two-fold isomorphisms, as far as we know, first appeared in literature in the form of isotopies of digraphs \cite{zelinka4}, \cite{zelinka1}, \cite{zelinka2}, \cite{zelinka3} and later studied formally in \cite{lms1}, \cite{lms2} with a greater emphasis on undirected graphs. We then turn our attention to the stability of graphs which have every edge on a triangle, but with the fresh outlook provided by TF-automorphisms. Amongst such graphs are strongly regular graphs with certain parameters. The advantages of this fresh outlook are highlighted when we ultimately present a method of constructing and generating unstable graphs with large diameter having every edge lying on a triangle. This was a rather surprising outcome.

math.CO↗

arXiv2014

Constrained colouring and $σ$-hypergraphs

A constrained colouring or, more specifically, an $(α,β)$-colouring of a hypergraph $H$, is an assignment of colours to its vertices such that no edge of $H$ contains less than $α$ or more than $β$ vertices with different colours. This notion, introduced by B{ú}jtas and Tuza, generalises both classical hypergraph colourings and the more general Voloshin colourings of hypergraphs. In fact, for $r$-uniform hypergraphs, classical colourings correspond to $(2,r)$-colourings while an important instance of Voloshin colourings of $r$-uniform hypergraphs gives $(2, r-1)$-colourings. One intriguing aspect of all these colourings, not present in classical colourings, is that $H$ can have gaps in its $(α,β)$-spectrum, that is, for $k_1 < k_2 < k_3$, $H$ would be $(α,β)$-colourable using $k_1$ and using $k_3$ colours, but not using $k_2$ colours. In an earlier paper, the first two authors introduced, for $σ$ a partition of $r$, a very versatile type of $r$-uniform hypergraph which they called $σ$-hypergraphs. They showed that, by simple manipulation of the parameters of a $σ$-hypergraph $H$, one can obtain families of hypergraphs which have $(2,r-1)$-colourings exhibiting various interesting chromatic properties. They also showed that, if the smallest part of $σ$ is at least 2, then $H$ will never have a gap in its $(2,r-1)$-spectrum but, quite surprisingly, they found examples where gaps re-appear when $α=β=2$. In this paper we extend many of the results of the first two authors to more general $(α,β)$-colourings, and we study the phenomenon of the disappearanace and re-appearance of gaps and show that it is not just the behaviour of a particular example but we place it within the context of a more general study of constrained colourings of $σ$-hypergraphs.

math.CO↗

arXiv2013

On the edge-reconstruction number of a tree

The edge-reconstruction number ern$(G)$ of a graph $G$ is equal to the minimum number of edge-deleted subgraphs $G-e$ of $G$ which are sufficient to determine $G$ up to isomorphsim. Building upon the work of Molina and using results from computer searches by Rivshin and more recent ones which we carried out, we show that, apart from three known exceptions, all bicentroidal trees have edge-reconstruction number equal to 2. We also exhibit the known trees having edge-reconstruction number equal to 3 and we conjecture that the three infinite families of unicentroidal trees which we have found to have edge-reconstruction number equal to 3 are the only ones.

math.CO↗

arXiv2013

Non-monochromatic non-rainbow colourings of $σ$-hypergraphs

One of the most interesting new developments in hypergraph colourings in these last few years has been Voloshin's notion of colourings of mixed hypergraphs. In this paper we shall study a specific instance of Voloshin's idea: a non-monochromatic non-rainbow (NMNR) colouring of a hypergraph is a colouring of its vertices such that every edge has at least two vertices coloured with different colours (non-monochromatic) and no edge has all of its vertices coloured with distinct colours (non-rainbow). Perhaps the most intriguing phenomenon of such colourings is that a hypergraph can have gaps in its NMNR chromatic spectrum, that is, for some $k_1 < k_2 < k_3$, the hypergraph is NMNR colourable with $k_1$ and with $k_3$ colours but not with $k_2$ colours. Several beautiful examples have been constructed of NMNR colourings of hypergraphs exhibiting phenomena not seen in classical colourings. Many of these examples are either \emph{ad hoc} or else are based on designs. The latter are difficult to construct and they generally give uniform $r$-hypergraphs only for low values of $r$, generally $r=3$. In this paper we shall study the NMNR colourings of a type of $r$-uniform hypergraph which we call $σ$-hypergraphs. The attractive feature of these $σ$-hypergraphs is that they are easy to define, even for large $r$, and that, by suitable modifications of their parameters, they can give families of hypergraphs which are guaranteed to have NMNR spectra with gaps or NMNR spectra without gaps. These $σ$-hypergraphs also team up very well with the notion of colour-bounded hypergraphs recently introduced by Bujt{á}s and Tuza to give further control on the appearance of gaps and perhaps explain better the existence of gaps in the colouring of mixed hypergraphs.

math.CO↗